Linux formula should be able to create non-unique users.

Change-Id: I3338950ed3726a750149d849d33813b0eeed50c0

README.rst

             home: '/home/jdoe'
             home_dir_mode: 755
             email: 'jonh@doe.com'
+            unique: false
           jsmith:
             name: 'jsmith'
             enabled: true

linux/system/user.sls

   {%- if user.uid is defined and user.uid %}
   - uid: {{ user.uid }}
   {%- endif %}
+  {%- if user.unique is defined %}
+  - unique: {{ user.unique }}
+  {%- endif %}
   {%- if user.maxdays is defined %}
   - maxdays: {{ user.maxdays }}
   {%- endif %}

tests/pillar/system.sls

         uid: 9999
         full_name: Test User
         home: /home/test
+        unique: false
         groups:
           - db-ops
           - salt-ops