[master] created API and test to query regionserver string parameters

Author: edgan

Summary:
Allows HTable clients to retrive configuration properties from the regionserver by
pinging a random server, either from its servercache or by querying the meta table

Test Plan: See attached unit test, which passes

Reviewers: manukranthk

Reviewed By: manukranthk

CC: clayton, hbase-eng@

Differential Revision: https://phabricator.fb.com/D956490

Task ID: 2730405

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.89-fb@1522689 13f79535-47bb-0310-9956-ffa450edef68