fix ami-type flag

[ndbaker1]

Issue #, if available:

Description of changes:

missed a test to validate this flag, and looks like it can only parse under the string primitive and not the ekstypes.AMITypes wrapper type

Before:

$ kubetest2 eksapi --ami-type=FOO
Error: unknown flag: --ami-type

Usage:
  kubetest2 eksapi [Flags] [DeployerFlags] -- [TesterArgs]

Flags:
      --artifacts string         top-level directory to put artifacts under for each kubetest2 run, defaulting to "${ARTIFACTS:-./_artifacts}". If using the ginkgo tester, this must be an absolute path. (default "/home/nbakerd/workspace/aws-k8s-tester/kubetest2/_artifacts")
      --build                    build kubernetes
      --down                     tear down the test cluster
  -h, --help                     display help
      --run-id string            unique identifier for a kubetest2 run (default "60358132-dc7f-467f-8d03-69da7996ed21")
      --rundir string            directory to put run related test binaries like e2e.test, ginkgo, kubectl for each kubetest2 run, defaulting to "${KUBETEST2_RUN_DIR:-./_rundir}". If using the ginkgo tester, this must be an absolute path.
      --rundir-in-artifacts      if true, the test binaries and run specific metadata will be in the ARTIFACTS
      --skip-test-junit-report   skip reporting the test step as a JUnit test case, should be set to true when solely relying on the tester binary to generate it's own junit.
      --test string              test type to run, if unset no tests will run
      --up                       provision the test cluster

DeployerFlags(eksapi):
      --add_dir_header                          If true, adds the file directory to the header
      --addons strings                          Managed addons (name:version pairs) to create in the cluster. Use 'latest' for the most recent version, or 'default' for the default version. (default [])
      --alsologtostderr                         log to standard error as well as files
      --ami string                              AMI for unmanaged nodes
      --cluster-role-service-principal string   Additional service principal that can assume the cluster role
      --efa                                     Create EFA interfaces on the node of an unmanaged nodegroup. Requires --unmanaged-nodes.
      --emit-metrics                            Record and emit metrics to CloudWatch
      --endpoint-url string                     Endpoint URL for the EKS API
      --expected-ami string                     Expected AMI of nodes. Up will fail if the actual nodes are not utilizing the expected AMI. Defaults to --ami if defined.
      --generate-ssh-key                        Generate an SSH key to use for tests. The generated key should not be used in production, as it will not have a passphrase.
      --instance-types strings                  Node instance types (default [])
      --ip-family string                        IP family for the cluster (ipv4 or ipv6)
      --kubeconfig string                       Path to kubeconfig
      --kubernetes-version string               cluster Kubernetes version
      --log_backtrace_at traceLocation          when logging hits line file:N, emit a stack trace (default :0)
      --log_dir string                          If non-empty, write log files in this directory
      --log_file string                         If non-empty, use this log file
      --log_file_max_size uint                  Defines the maximum size a log file can grow to. Unit is megabytes. If the value is 0, the maximum file size is unlimited. (default 1800)
      --logtostderr                             log to standard error instead of files (default true)
      --node-name-strategy string               Specifies the naming strategy for node. Allowed values: ['SessionName', 'EC2PrivateDNSName'], default to EC2PrivateDNSName
      --node-ready-timeout duration             Time to wait for all nodes to become ready (default 0s)
      --nodes int                               number of nodes to launch in cluster
      --region string                           AWS region for EKS cluster
      --skip_headers                            If true, avoid header prefixes in the log messages
      --skip_log_headers                        If true, avoid headers when opening log files
      --stderrthreshold severity                logs at or above this threshold go to stderr (default 2)
      --tune-vpc-cni                            Apply tuning parameters to the VPC CNI DaemonSet
      --unmanaged-nodes                         Use an AutoScalingGroup instead of an EKS-managed nodegroup. Requires --ami
      --up-cluster-header strings               Additional header to add to eks:CreateCluster requests. Specified in the same format as curl's -H flag. (default [])
      --user-data-format string                 Format of the node instance user data
  -v, --v Level                                 number for the log level verbosity
      --vmodule moduleSpec                      comma-separated list of pattern=N settings for file-filtered logging

After:

$ kubetest2 eksapi --ami-type=FOO
I0412 01:48:48.357699 1412239 app.go:61] The files in RunDir shall not be part of Artifacts
I0412 01:48:48.357738 1412239 app.go:62] pass rundir-in-artifacts flag True for RunDir to be part of Artifacts
I0412 01:48:48.357752 1412239 app.go:64] RunDir for this run: "/home/nbakerd/workspace/aws-k8s-tester/kubetest2/_rundir/a4371ae8-02d9-4947-8c33-ebb4170e5684"
I0412 01:48:48.360867 1412239 app.go:136] ID for this run: "a4371ae8-02d9-4947-8c33-ebb4170e5684"

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.