Adding ability to set up optional groups for user.

Change-Id: I0109e9b5298253fac0dac090066c313e8f7fccbb
diff --git a/linux/system/user.sls b/linux/system/user.sls
index 42086d5..64636f3 100644
--- a/linux/system/user.sls
+++ b/linux/system/user.sls
@@ -45,6 +45,9 @@
   {%- if user.groups is defined %}
   - groups: {{ user.groups }}
   {%- endif %}
+  {%- if user.optional_groups is defined %}
+  - optional_groups: {{ user.optional_groups }}
+  {%- endif %}
   {%- if user.system is defined and user.system %}
   - system: True
   - shell: {{ user.get('shell', '/bin/false') }}