[SPARK-5619][SQL] Support 'show roles' in HiveContext

Author: q00251598 <qiyadong@huawei.com>

Closes #4397 from watermen/SPARK-5619 and squashes the following commits:

f819b6c [q00251598] Support show roles in HiveContext.

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

@@ -68,6 +68,7 @@ private[hive] object HiveQl {
     "TOK_SHOWLOCKS",
     "TOK_UNLOCKTABLE",
 
+    "TOK_SHOW_ROLES",
     "TOK_CREATEROLE",
     "TOK_DROPROLE",
     "TOK_GRANT",