Include GPG keys in katello-repos and check for non-nightly

[ekohl]

Including the GPG keys is better because it gives a higher degree of trust because the RPM is GPG signed. It also helps because dnf doesn't refresh HTTPS URLs and purging old entries is tedious. While the repos aren't typically installed that long, I've had it happen to me with one of my personal COPR repositories.

The downside is that after branching you need to update the GPG key in the repository.

It also makes GPG key checking correct by looking at the correct values. This allows enabling GPG for candlepin (which is using a signed release) while not enabling it for Pulp or Katello (which are nightly).