Fixes #6282 - consumer cert url of redhat_register

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, I was thinking that subscription_manager_host was manually entered so having the user enter the `subscription_manager_certpkg_url` shouldn't be a huge change for them. I guess I'm just echoing you question to others...Is there a need to fall back on the old param? 

@lzap, @domcleal, @jameslabocki, @witlessbird  do you know ?