Placement needs a configurable healthz port

[yaron2]

Currently, the healthz port for the placement service is always 8080 with no configurable option to set the port.
This can cause the placement service to throw an error when running on a machine where port 8080 is taken.

First reported by @samcov here.

The error does not prevent from the Placement Service to function normally.

Release Note

RELEASE NOTE: Added configurable healthz port for placement service

[yaron2]

Update: we log.Fatal for failing to start the healthz server which exists the process, which makes the placement service exit if a healthz port is already used.