mgr/cephadm: only try to place mons on hosts matching public_network

mgr/cephadm: only try to place mons on hosts matching public_network

Only try to schedule new mons on hosts that match the configured
public_network, if any.  If we do not have one configured, then don't
try to place new mons at all.

Note that there are other restrictions that ceph-mon supports that we
aren't considering, here: public_network_interface in particular, which
might further limit which IPs we consider binding to.

Signed-off-by: Sage Weil <sage@redhat.com>