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.
Since we want users to migrate to the new subscription_manager_certpkg_url, I'm not sure this additional parameter is needed. If users are changing their parameter anyway, they might as well migrate to the new one.