core, restapi : Add /tags sub-collection for Template resource

Add ability to add/retrieve and delete tags for
templates using rest api.

Change-Id: I32e2d449f7154c50bc7511e143ed82c995a3fa4e
Bug-Url: https://bugzilla.redhat.com/852086
Signed-off-by: Ravi Nori <rnori@redhat.com>