restapi: 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=1011735
Change-Id: I95261d17138a8a19d52b16f7f84322c051d7bf17
Signed-off-by: Omer Frenkel <ofrenkel@redhat.com>