extend cephadm to do minimal client setup for kcephfs (and maybe krbd)?

I've had this PR sitting around for a while: https://github.com/ceph/ceph/pull/31885 It's bitrotted a bit, and I'll clean that up soon, but after looking over cephadm, I wonder if it would make sense to also extend it to do these actions on machines that are just intended to be kcephfs or krbd clients. We typically don't need to do a full-blown install on the clients, so being able to install just the minimum packages needed and do a minimal conf/keyring setup would be nice. Does this make sense? I'll open a tracker if the principal cephadm devs are OK with it. Thanks, -- Jeff Layton <jlayton(a)redhat.com>