Delete the default guest user from rabbitmq

Leaving the default user enabled is a security issue, as it can be used
without credentials. It also may mask issues like seen in [1].

[1] https://bugs.launchpad.net/bugs/1651576

Change-Id: I75b4e5696c0f8017b869127a10f3c14e2f8bd121

lib/rpc_backend

@@ -97,6 +97,8 @@ function restart_rpc_backend {
 
             break
         done
+        # NOTE(frickler): Remove the default guest user
+        sudo rabbitmqctl delete_user guest || true
     fi
 }