mgr/cephadm: fix handling of draining hosts with explicit placement specs

mgr/cephadm: fix handling of draining hosts with explicit placement specs

Basically, if you have a placement that explicitly defines the hosts
to place on, and then add _no_schedule label to one of the hosts (which
should cause all daemons to be removed from the host) cpehadm will simply
fail to apply the spec, saying the host with the _no_schedule label is "Unknown".
This is due to the fact that we remove hosts with the _no_schedule label from
the pool of hosts the scheduler has to work with entirely. If we also provide
the scheduler with a list of currently draining hosts, it can handle this
better and the daemon can be drained off the host as expected.

Fixes: https://tracker.ceph.com/issues/56972
Signed-off-by: Adam King <adking@redhat.com>