Prevent invalid policy_arns from being created.

[benashz]

With the update to TF SDKv2 calling schema.ResourceData.Get() on a
TypeSet now returns nil. The previous code was testing for empty string,
which resulted in the policy_arns being set to [""] in Vault when no
policy_arns were specified in the TF code.

The offending conditional was referencing the vestigial schema key named
policy_arn, which has now been removed.

Additional fixes:

• remove vestigial schema key policy
• extend tests to cover reported failure mode
• enhance other policy_arn tests
• always validate the JSON policy document to avoid invalid input
• update util.JsonDiffSuppress to log at the warn level rather than
  error, since the JSON parse errors are treated as non fatal

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Closes #1226 #925

Output from acceptance testing:

$ make dev testacc TESTARGS='-v -test.run TestAccAWSSecretBackendRole'
==> Checking that code complies with gofmt requirements...
go build -o terraform-provider-vault
mv terraform-provider-vault ~/.terraform.d/plugins/
TF_ACC=1 go test $(go list ./...) -v -v -test.run TestAccAWSSecretBackendRole -timeout 20m

[...]

=== RUN   TestAccAWSSecretBackendRole_basic
--- PASS: TestAccAWSSecretBackendRole_basic (4.77s)
=== RUN   TestAccAWSSecretBackendRole_import
--- PASS: TestAccAWSSecretBackendRole_import (3.62s)
=== RUN   TestAccAWSSecretBackendRole_nested
--- PASS: TestAccAWSSecretBackendRole_nested (3.40s)
PASS
ok      github.com/hashicorp/terraform-provider-vault/vault     (cached)
...

[vinay-gopalan]

LGTM!