mgr/cephadm: teach serve.py about host selector support

mgr/cephadm: teach serve.py about host selector support

A previous commit added a HostSelector protocol type to the schedule
code. This change makes it so the function calling upon the
HostAssignment class detects if a CephService provides a
filter_host_candidates method - meaning the service class can act as a
HostSelector. If the class can be a HostSelector pass it to the
HostAssignment so that the custom selection operation can be run.

Signed-off-by: John Mulligan <jmulligan@redhat.com>