Fix AWS Secrets Role to allow IAM Groups only

[lawliet89]

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Release note for CHANGELOG:

- Fix AWS Secrets Engine Role resource to allow only IAM Groups

Output from acceptance testing:

$ AWS_ACCESS_KEY_ID=1234 AWS_SECRET_ACCESS_KEY=4321 VAULT_ADDR='http://127.0.0.1:8200' VAULT_TOKEN=12345 make testacc TESTARGS="-run=TestAccAWSSecretBackendRole"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run=TestAccAWSSecretBackendRole -timeout 120m
?       github.com/terraform-providers/terraform-provider-vault [no test files]
?       github.com/terraform-providers/terraform-provider-vault/cmd/coverage    [no test files]
?       github.com/terraform-providers/terraform-provider-vault/cmd/generate    [no test files]
testing: warning: no tests to run
PASS
ok      github.com/terraform-providers/terraform-provider-vault/codegen 0.079s [no tests to run]
?       github.com/terraform-providers/terraform-provider-vault/generated       [no test files]
testing: warning: no tests to run
PASS
ok      github.com/terraform-providers/terraform-provider-vault/generated/datasources/transform/decode  0.053s [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/terraform-providers/terraform-provider-vault/generated/datasources/transform/encode  0.073s [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/terraform-providers/terraform-provider-vault/generated/resources/transform/alphabet  0.032s [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/terraform-providers/terraform-provider-vault/generated/resources/transform/role      0.060s [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/terraform-providers/terraform-provider-vault/generated/resources/transform/template  0.045s [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/terraform-providers/terraform-provider-vault/generated/resources/transform/transformation    0.025s [no tests to run]
?       github.com/terraform-providers/terraform-provider-vault/schema  [no test files]
testing: warning: no tests to run
PASS
ok      github.com/terraform-providers/terraform-provider-vault/util    0.029s [no tests to run]
=== RUN   TestAccAWSSecretBackendRole_basic
--- PASS: TestAccAWSSecretBackendRole_basic (0.54s)
=== RUN   TestAccAWSSecretBackendRole_import
--- PASS: TestAccAWSSecretBackendRole_import (0.34s)
=== RUN   TestAccAWSSecretBackendRole_nested
--- PASS: TestAccAWSSecretBackendRole_nested (0.36s)
PASS
ok      github.com/terraform-providers/terraform-provider-vault/vault   1.269s

[catsby]

LGTM 👍 -- Failing test is an unrelated flaky test