core: add custom properties to 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: I85f8fb998b5e081c10d93e5790963fb7b8bdf5fd
Signed-off-by: Omer Frenkel <ofrenkel@redhat.com>