frontend: support for making sub-templates

Allows the user to create template as sub-template of other template in
the data center from both webadmin and user-portal

http://www.ovirt.org/index.php?title=Features/Template_Versions

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1037478
Change-Id: Ic35174ac6736b96e8854885a711830c93a4b79ad
Signed-off-by: Arik Hadas <ahadas@redhat.com>