Add Katello support for RHVH/oVirt Node provisioning (#466)

• Add Katello support for RHVH/oVirt Node provisioning

Leverages `repository_url` helper to generate full url to
Katello-provided squashfs img, based on relative path.

• Refactor the ovirt template to use redhat_register snippet

Also, eliminates the number of parameters needed to be set to specify
the relative path to the liveimg image, reusing the liveimg_name
for the Katello use case.