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

(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.

Trace how /cephadm_bootstrap/mon_ip is read and passed to cephadm bootstrap, including the current fallback that selects an address from the bootstrap node. Done means an unset value is rejected and a configured value is verified to exist on the bootstrap node.

Written by the indexing model from the issue text.