HIVE-5931 : SQL std auth - add metastore get_principals_in_role api, support SHOW PRINCIPALS role_name (Thejas Nair via Ashutosh Chauhan)

git-svn-id: https://svn.apache.org/repos/asf/hive/branches/branch-0.13@1576677 13f79535-47bb-0310-9956-ffa450edef68