mgr/cephadm: prefer same hosts as related service daemons when picking arbitrary...

mgr/cephadm: prefer same hosts as related service daemons when picking arbitrary hosts

For now, just for linking ingress services and
their backend services. The idea is if one, or both,
of the ingress service and backend service is using a
count, to try and get them to deploy their daemons
on the same host(s). If the placements have explicit
placements (not using count) we still stick to
those placements regardless.

This should enable something like specifying a host
for the backend service and leaving the ingress
placement as just "count: 1" and having the ingress
service get on the same host as the backend service
daemon. This is particularly useful for the keepalive-only
(VIP but no haproxy) over NFS setup where the keepalive
must share a host with the NFS to function, but will
also be useful for other VIP only setups we may do
in the future.

Signed-off-by: Adam King <adking@redhat.com>