[HBASE-8587] Added a utility to make the HMaster reload its configuration from disk

Author: gauravm

Summary:
We have the ability to make the RegionServers reload their configurations, this diff will allow us to make the HMaster also reload its configuration.

The next step would involve making all the relevant classes observers and implementing their notifyOnChange() methods.

Test Plan: Unit Tests

Reviewers: liyintang, rshroff, manukranthk, adela, shaneh, aaiyer

Reviewed By: aaiyer

CC: hbase-eng@

Differential Revision: https://phabricator.fb.com/D820099

Task ID: 2258346

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.89-fb@1485319 13f79535-47bb-0310-9956-ffa450edef68