Avoid duplicates of the 'all' group when using inventory script.

Do not recreate an 'all' group when it is in the inventory script's
output, but use the one created upfront.

lib/ansible/inventory/script.py

@@ -72,7 +72,10 @@ class InventoryScript(object):
                     self.host_vars_from_top = data['hostvars']
                     continue
 
-            group = groups[group_name] = Group(group_name)
+            if group_name != all.name:
+                group = groups[group_name] = Group(group_name)
+            else:
+                group = all
             host = None
 
             if not isinstance(data, dict):