Adds os_admin_user to common OS image prop metadef

This will add os_admin_user to the list of common properties for
operating systems metadata definition.

Commit I215bb82668df074141397909b58fa9189f46b4da is adding
os_admin_user to the list of image metadata it understands.  As
a result, it's beneficial to also have that property defined
in the metadef.

Change-Id: I7cf70acf3c6a4f801b34d52926c444d05b79fd51
Implements: blueprint libvirt-set-admin-password

etc/metadefs/operating-system.json

@@ -22,6 +22,11 @@
             "title": "OS Version",
             "description": "Operating system version as specified by the distributor. (for example, '11.10')",
             "type": "string"
+        },
+        "os_admin_user": {
+            "title": "OS Admin User",
+            "description": "The name of the user with admin privileges.",
+            "type": "string"
         }
     }
 }