Change group to optional_group

The patch change the group parameter to optional_group
due to avoid issue during highstate on compute nodes.

Change-Id: I9a808d9c480f7a5fcd61d034b284690df6d0386d
diff --git a/ceilometer/agent.sls b/ceilometer/agent.sls
index 72bbfad..2e075a8 100644
--- a/ceilometer/agent.sls
+++ b/ceilometer/agent.sls
@@ -15,11 +15,11 @@
   - require:
     - pkg: ceilometer_agent_packages
 
-{%- if agent.get('libvirt',{}).get('ssl',{}).get('enabled', False) == True  and salt['group.info']('nova') %}
+{%- if agent.get('libvirt',{}).get('ssl',{}).get('enabled', False) == True %}
 add_ceilometer_to_nova_group:
   user.present:
   - name: ceilometer
-  - groups:
+  - optional_groups:
     - nova
   - require:
     - pkg: ceilometer_agent_packages