Fix plugins in HA mode

Another problem with HA rabbitmq plugins. Since we run start_app
and stop_app several times, plugins re-setup() causes problems.
So we just need to check if plugins are already loaded before
trying to setup() them again.

Change-Id: I4e13a405b6895f92d3da3ac0fd69274951837e4e
Closes-bug: #1354026