[SPARK-18662] Move resource managers to separate directory

## What changes were proposed in this pull request?

* Moves yarn and mesos scheduler backends to resource-managers/ sub-directory (in preparation for https://issues.apache.org/jira/browse/SPARK-18278)
* Corresponding change in top-level pom.xml.

Ref: https://github.com/apache/spark/pull/16061#issuecomment-263649340

## How was this patch tested?

* Manual tests

/cc rxin

Author: Anirudh <ramanathana@google.com>

Closes #16092 from foxish/fix-scheduler-structure-2.