Limit agent restarts preventing pollers from going out of sync (#393)

* Limit agent restarts to help prevent private pollers from getting out of sync (temporary workaround)

* Add maas_restart_serial for overriding this value

(cherry picked from commit 7e32dd1)

playbooks/maas-restart.yml

@@ -16,6 +16,7 @@
 - name: Restart MaaS
   hosts: "{{ maas_restart_on_host_override | default('maas_restart_on_host') }}"
   gather_facts: false
+  serial: "{{ maas_restart_serial | default(1) }}"
   tasks:
     - name: Restart rackspace-monitoring-poller (systemd)
       service: