fix: Handle case when IAM Policy was deleted externally

[s-vitaliy]

Handle case when an IAM policy was Removed externally

Currently the provider has very annoying bug: if an IAM policy was removed externally, plan fails and the project that contains minio policies becomes unplannable.

This PR implements the following changes:

• Added code that handles the XMinioAdminNoSuchPolicy error code, similar to how it is handled in the minio_iam_user resource.
• Added a unit test validating the corrected behavior

The new behavior:

$ terraform plan 
minio_iam_policy.test_policy: Refreshing state... [id=state-terraform-s3]

Note: Objects have changed outside of Terraform

Terraform detected the following changes made outside of Terraform since the last "terraform apply":

  # minio_iam_policy.test_policy has been deleted
  - resource "minio_iam_policy" "test_policy" {
      - id     = "state-terraform-s3" -> null
      - name   = "state-terraform-s3" -> null
      - policy = jsonencode(
            {
              - Statement = [
                  - {
                      - Action   = [
                          - "s3:GetBucketLocation",
                          - "s3:ListAllMyBuckets",
                        ]
                      - Effect   = "Allow"
                      - Resource = [
                          - "arn:aws:s3:::*",
                        ]
                      - Sid      = "1"
                    },
                  - {
                      - Action    = [
                          - "s3:*",
                        ]
                      - Condition = {
                          - StringLike = {
                              - s3:prefix = [
                                  - "",
                                  - "home/",
                                ]
                            }
                        }
                      - Effect    = "Allow"
                      - Resource  = [
                          - "arn:aws:s3:::state-terraform-s3",
                        ]
                    },
                  - {
                      - Action   = [
                          - "s3:DeleteObject",
                          - "s3:PutObject",
                        ]
                      - Effect   = "Allow"
                      - Resource = [
                          - "arn:aws:s3:::state-terraform-s3",
                          - "arn:aws:s3:::state-terraform-s3/*",
                        ]
                    },
                ]
              - Version   = "2012-10-17"
            }
        ) -> null
    }


Unless you have made equivalent changes to your configuration, or ignored the relevant attributes using ignore_changes, the following plan may include actions to undo or respond to these changes.

─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────

Terraform used the selected providers to generate the following execution plan. Resource actions are indicated with the following symbols:
  + create

Terraform will perform the following actions:

  # minio_iam_policy.test_policy will be created
  + resource "minio_iam_policy" "test_policy" {
      + id     = (known after apply)
      + name   = "state-terraform-s3"
      + policy = jsonencode(
            {
              + Statement = [
                  + {
                      + Action   = [
                          + "s3:ListAllMyBuckets",
                          + "s3:GetBucketLocation",
                        ]
                      + Effect   = "Allow"
                      + Resource = "arn:aws:s3:::*"
                      + Sid      = "1"
                    },
                  + {
                      + Action    = "s3:*"
                      + Condition = {
                          + StringLike = {
                              + s3:prefix = [
                                  + "",
                                  + "home/",
                                ]
                            }
                        }
                      + Effect    = "Allow"
                      + Resource  = "arn:aws:s3:::state-terraform-s3"
                      + Sid       = ""
                    },
                  + {
                      + Action   = [
                          + "s3:PutObject",
                          + "s3:DeleteObject",
                        ]
                      + Effect   = "Allow"
                      + Resource = [
                          + "arn:aws:s3:::state-terraform-s3/*",
                          + "arn:aws:s3:::state-terraform-s3",
                        ]
                      + Sid      = ""
                    },
                ]
              + Version   = "2012-10-17"
            }
        )
    }

Plan: 1 to add, 0 to change, 0 to destroy.

Changes to Outputs:
  ~ minio_id = "state-terraform-s3" -> (known after apply)

Reference

Resolves #550

Closing issues

• Closes: [FATAL] unable to read policy: The canned policy does not exist #550

[felladrin]

Thanks for the fix and congrats on your first contribution to this repo, @s-vitaliy! 🎉