Fixes #6282 - consumer cert url of redhat_register

DON'T MERGE BEFORE SQUASH
Fixes the url in the redhat_register snippet. Url was using the old url
used by katello. Snippet will use the katello configuration,
consumer_cert_rpm.
@ekohl updated, template allows users to continue to use `subscription_manager_host`. The user can also supply a `katello_historical` in cases where they want to use the old cert name.