frontend: support custom properties for templates

currently custom properties are vm only feature,
so adding also to templates.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1166632
Change-Id: If5d982307a9ffcee758dfec712916dc47ce3aab5
Signed-off-by: Omer Frenkel <ofrenkel@redhat.com>