ssh server: make listening port configurable

Make the listening port of sshd configurable,
so we can be more flexible. Keep 22 as default.

Triggered by the comments on Id27657ca29e419 where
we want to have a setup with Gerrit listening on 22
and multiple IPs.

Change-Id: Iefbe62a2489cda577541fed651c5ccbf7effc029
ssh server: make ListenAddress configurable

for situations like Id27657ca29e4196088

Bug: 35611
RT: 8838
Change-Id: Icb4cbadb2d3766869dbc5310121b69fc9e450bf2
Merge changes Icb4cbadb,Iefbe62a2 into production

* changes:
  ssh server: make ListenAddress configurable
  ssh server: make listening port configurable