Add admin endpoint for updating the configuration on the regionserver

Summary: This is a follow up diff for the jira to add the functionality to update configs in an online fashion.

Test Plan: Unit test. This adds an end point on the regionserver. It just tries to call the end point and make sure the call succeeds.

Differential Revision: https://reviews.facebook.net/D24861

Signed-off-by: stack <stack@apache.org>

Conflicts:
	hbase-server/src/test/java/org/apache/hadoop/hbase/regionserver/TestRegionServerOnlineConfigChange.java