Enable discovery of AMs by containers

This is already discussed on the umbrella JIRA YARN-1489.

Copying some of my condensed summary from the design doc (section 3.2.10.3) of YARN-4692.

Even after the existing work in Work­preserving AM restart (Section 3.1.2 / YARN-1489), we still haven’t solved the problem of old running containers not knowing where the new AM starts running after the previous AM crashes. This is a specifically important problem to be solved for long running services where we’d like to avoid killing service containers when AMs fail­over. So far, we left this as a task for the apps, but solving it in YARN is much desirable. [(Task) This looks very much like service­-registry (YARN-913), but for app­containers to discover their own AMs.

Combining this requirement (of any container being able to find their AM across fail­overs) with those of services (to be able to find through DNS where a service container is running - YARN-4757) will put our registry scalability needs to be much higher than that of just service end­points. This calls for a more distributed solution for registry readers something that is discussed in the comments section of YARN-1489 and MAPREDUCE-6608.
See comment https://issues.apache.org/jira/browse/YARN-1489?focusedCommentId=13862359&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-13862359