core: attempt revert when failing to add template

When failing to add a template, delete the added template disks from the
db and attempt to delete the possibly added images from the storage to avoid
storage leak.

Change-Id: Ia0f40397f5fb9f7e1a3535796c07f5bed7bd5ac1
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=883921
Signed-off-by: Liron Aravot <laravot@redhat.com>