[alert-manager] Add new internal port and endpoint for retrieving incidents.
[alert-manager] Add new internal port and endpoint for retrieving incidents.

From within the skia-public cluster a request to http://alert-manager:9000/_/incidents
should return an array of Incidents serialized as JSON:

  https://skia.googlesource.com/buildbot/+/master/am/go/incident/incident.go#42

Bug: skia:8345
Change-Id: Iefa85b35a719ca182cc67de12fb855cbacba1978
Reviewed-on: https://skia-review.googlesource.com/150925
Auto-Submit: Joe Gregorio <b9cf2471ff6d33504cafe8e6f356b732b2cadced@google.com>
Reviewed-by: Kevin Lubick <7cdab2cfab351f23814786ba39716e90eed69047@google.com>
Commit-Queue: Kevin Lubick <7cdab2cfab351f23814786ba39716e90eed69047@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot