Properly set loopback_users via rabbitmq::loopback_users

Sorta resurrecting https://review.opendev.org/#/c/324016/ now that
voxpupuli/puppet-rabbitmq#699 merged.

This commit prevents a duplicate entry for loopback_users in the
rabbitmq.config file:

{loopback_users, [<<"guest">>]},
{loopback_users, []},

keeping only the expected empty array.

Note: not a 100% clean cherry pick due to tht path changes.

(cherry picked from commit 04a5937)

Change-Id: I0c291bf5611f02983c430179206556bae3b38736

puppet/services/rabbitmq.yaml

@@ -103,6 +103,7 @@ outputs:
             rabbitmq::delete_guest_user: false
             rabbitmq::wipe_db_on_cookie_change: true
             rabbitmq::port: 5672
+            rabbitmq::loopback_users: []
             rabbitmq::package_provider: yum
             rabbitmq::package_source: undef
             rabbitmq::repos_ensure: false
@@ -121,7 +122,6 @@ outputs:
             rabbitmq_config_variables:
               cluster_partition_handling: 'ignore'
               queue_master_locator: '<<"min-masters">>'
-              loopback_users: '[]'
             rabbitmq::erlang_cookie:
               yaql:
                 expression: $.data.passwords.where($ != '').first()