restapi: add template versions support

Add to templates REST resource a new section that
describes template version details.
The user will be able to add this in POST (add template),
update version name in PUT (update) and see version details in
GET requests.

Feature page:
http://www.ovirt.org/Features/Template_Versions

Change-Id: Id8e4c498c4a1856ac341237837db40db8375c6da
Signed-off-by: Alissa Bonas <abonas@redhat.com>