Added policy support to rabbitmq module

Added methods for:
 - list_policies
 - set_policy
 - delete_policy
 - policy_exists
Added state for checking rabbitmq policies are present or absent
Merge branch 'patch-1' into feature-rabbitmq_policy
Added reference url in comments of policy methods
Merge pull request #6946 from beichhor/feature-rabbitmq_policy

Added policy support for rabbitmq state and module