Install CNI Refactor

[bcheung]

Fixes #33 and #93

• Created an internal directory that holds the interfaces and mock testing tools for system/platform related dependencies (iptables, http requests, systctl etc.). This decouples the implementation logic from these dependencies so that the code is more unit-testable.
• Rewrote the install-cni.sh script in Golang and added unit tests for each plugin installer.

install-cni structure:

• The structure of install-cni program decouples the process (cmd/install-cni/main.go) from the root command (pkg/install-cni/cmd/root.go) and its related packages. The root command is where the binary gets all the environment variables and calls the CNIInstaller in pkg/install-cni/install.
• The CNI installer is where the script logic/flow is implemented and runs the plugin installers.
• The plugins are designed to be decoupled from each other and each plugin implements an Installer interface that requires a Run() function.

The last commit marks the remaining TODO items.

Note: These changes need to be tested with integration tests to ensure that it works with the internal GKE codebase.

[k8s-ci-robot]

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