webadmin: Add external user group to Quota consumers

Admin user cannot add a group of external users to
the consumers of a selected Quota.
This regression caused by commit: c68c60e153b7ffd1d900af7b00514b7940cceeb0

Change-Id: I1c1d82596c69f1ff4173f02c5dddf60317eeebb7
Bug-Url: https://bugzilla.redhat.com/1081014
Signed-off-by: Kobi Ianko <kobi@redhat.com>