[SPARK-5686][SQL] Add show current roles command in HiveQl

show current roles

Author: OopsOutOfMemory <victorshengli@126.com>

Closes #4471 from OopsOutOfMemory/show_current_role and squashes the following commits:

1c6b210 [OopsOutOfMemory] add show current roles

sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveQl.scala

@@ -77,6 +77,7 @@ private[hive] object HiveQl {
     "TOK_REVOKE",
     "TOK_SHOW_GRANT",
     "TOK_SHOW_ROLE_GRANT",
+    "TOK_SHOW_SET_ROLE",
 
     "TOK_CREATEFUNCTION",
     "TOK_DROPFUNCTION",