Preflight repo issues in airgapped ubuntu environments

[mboeker]

Running into several issues trying to run the preflight playbook in an ubuntu airgapped environment:

1. ceph_stable_key is always required, no matter what kind of repo. Our airgapped mirror works fine with just [trusted=yes] http://bla/path

2. Documentation says ceph custom repo and repo_url are mutually exclusive, they are not. If you define custom URL you also have to define the repo. Again, just the one line is enough.

3. Ceph custom repositories should be custom. The path has distribution_release hardcoded, should be defined to be custom. You can set components, but you can't set (or remove) distribution_release.

4. Docker is hardcoded to go to download.docker.com. Definitely not airgapped.