add support for setting arbitrary user tags on a rabbitmq_user

This commit adds support for setting arbitrary user tags on a rabbitmq_user.

Note that this creates potential conflicts with the admin property.  It tries to be smart about such conflicts.  But I would say that ideally, admin and user_tags should not be defined on the same object.

This is my first pull request.  Please let me know if I'm doing anything wrong.  Thanks!