site.yml: do not bootstrap mgrs on monitors by default

Let's bootstrap mgrs on monitors only if there's no mgrs section in
inventory hostfile.

Closes: #3613

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>

site-container.yml.sample

@@ -126,6 +126,7 @@
         name: ceph-mon
     - import_role:
         name: ceph-mgr
+      when: groups.get(mgr_group_name, []) | length == 0
 
 - hosts: mons
   any_errors_fatal: true

site.yml.sample

@@ -106,6 +106,7 @@
         name: ceph-mon
     - import_role:
         name: ceph-mgr
+      when: groups.get(mgr_group_name, []) | length == 0
 
   post_tasks:
     - name: set ceph monitor install 'Complete'