Change SASL server GSSAPI callback log line from DEBUG to TRACE in RegionServer to reduce log volumes in DEBUG mode

For every client connect call to RS we are logging below log line in DEBUG mode which is causing too many logs in RegionServer.

2017-01-18 17:51:40,739 DEBUG [.reader=4,port=60020] security.HBaseSaslRpcServer - SASL server GSSAPI callback: setting canonicalized client ID: hbase/hostname-xxx@KDC