Run service_setup only once

There's no reason to run service_setup for each magnum container
as it's enough to be done once. So we're including role only
when it's first host in the play.

Change-Id: I8f282d4a284e0de395081251f6622df0c1daeeb3

tasks/magnum_service_setup.yml

@@ -68,5 +68,7 @@
       - name: "{{ magnum_service_name }}"
         type: "{{ magnum_service_type }}"
         description: "{{ magnum_service_description }}"
+  when:
+    - "_magnum_is_first_play_host"
   tags:
     - always