The `iam` command can be forced to use the XML API, even though it's not supported

Context: https://stackoverflow.com/questions/56958985/got-failure-getbucketiampolicy-must-be-overloaded-when-gsutil-iam-get-gs-te

If HMAC credentials are the only creds configured, gsutil will force a command to use the XML API (since HMAC credentials only work with that API).

We added a check for this to the `kms` command, but apparently missed adding it to the `iam` command:
https://github.com/GoogleCloudPlatform/gsutil/blob/0e4bdc80f90f42edd86c3da772c22087e63b21be/gslib/commands/kms.py#L384

A similar check should be added to `RunCommand` in `gslib/commands/iam.py`.