conditionally avoid installing from ceph repos

This pull request adds a flag to avoid the behavior of always using ceph.com repos for all distro installations.

For some users/organizations this is annoying to say the least because if they control their own repos or are behind a firewall they cannot override this behavior.

With the new flag (`--pkgs-only`) a ceph-deploy user will be able to skip the repo mangling and just install pkgs.