Force user to populate "/cephadm_bootstrap/mon_ip" with a real IP address

[smithfarm]

(Please correct me if I'm wrong about any of this!)

When the user leaves /cephadm_bootstrap/mon_ip unset, ceph-salt's current behavior is to randomly choose one of the IP addresses of the bootstrap node, and provide that to cephadm bootstrap (which does require an explicit --mon-ip option).

This is wrong, because it enables the user to easily overlook that the setting of /cephadm_bootstrap/mon_ip determines not just the IP address that the MON listens on, but also the public network of the whole cluster.

Therefore, ceph-salt should force the user to set cephadm_boostrap/mon_ip explicitly.

It should also verify that the IP address exists on the bootstrap node.