Update user module to support group name

This changes the gid option to group.  One may provide a primary group
as either a gid or a name.  The module will then check to verify that
the group already exists.  If the group does not already exist, the
module will fail.

examples/playbooks/user_commands.yml

@@ -13,7 +13,7 @@
 
     # Walk through account creation, modification, and deletion
     - name: test basic user account creation
-      action: user name=tset comment=TsetUser gid=100 shell=/sbin/nologin createhome=no
+      action: user name=tset comment=TsetUser group=100 shell=/sbin/nologin createhome=no
 
     # the following is just a simple example of how you don't have to include
     # the 'name' element for each task