[SPARK-2817] add "show create table" support

Author: tianyi

sql/hive/compatibility/src/test/scala/org/apache/spark/sql/hive/execution/HiveCompatibilitySuite.scala

@@ -635,6 +635,14 @@ class HiveCompatibilitySuite extends HiveQueryFileTest with BeforeAndAfter {
     "serde_regex",
     "serde_reported_schema",
     "set_variable_sub",
+    "show_create_table_partitioned",
+    "show_create_table_delimited",
+    "show_create_table_alter",
+    "show_create_table_view",
+    "show_create_table_serde",
+    "show_create_table_db_table",
+    "show_create_table_does_not_exist",
+    "show_create_table_index",
     "show_describe_func_quotes",
     "show_functions",
     "show_partitions",

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

@@ -51,6 +51,7 @@ private[hive] object HiveQl {
   protected val nativeCommands = Seq(
     "TOK_DESCFUNCTION",
     "TOK_DESCDATABASE",
+    "TOK_SHOW_CREATETABLE",
     "TOK_SHOW_TABLESTATUS",
     "TOK_SHOWDATABASES",
     "TOK_SHOWFUNCTIONS",