[SPARK-5591][SQL] Fix NoSuchObjectException for CTAS

Now CTAS runs successfully but will throw a NoSuchObjectException.
```
create table sc as select *
from (select '2011-01-11', '2011-01-11+14:18:26' from src tablesample (1 rows)
union all
select '2011-01-11', '2011-01-11+15:18:26' from src tablesample (1 rows)
union all
select '2011-01-11', '2011-01-11+16:18:26' from src tablesample (1 rows) ) s;
```
Get this exception:
ERROR Hive: NoSuchObjectException(message:default.sc table not found)
at org.apache.hadoop.hive.metastore.HiveMetaStore$HMSHandler.get_table(HiveMetaStore.java:1560)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:601)
at org.apache.hadoop.hive.metastore.RetryingHMSHandler.invoke(RetryingHMSHandler.java:105)
at $Proxy8.get_table(Unknown Source)
at org.apache.hadoop.hive.metastore.HiveMetaStoreClient.getTable(HiveMetaStoreClient.java:997)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:601)
at org.apache.hadoop.hive.metastore.RetryingMetaStoreClient.invoke(RetryingMetaStoreClient.java:89)
at $Proxy9.getTable(Unknown Source)
at org.apache.hadoop.hive.ql.metadata.Hive.getTable(Hive.java:976)
at org.apache.hadoop.hive.ql.metadata.Hive.getTable(Hive.java:950)
at org.apache.spark.sql.hive.HiveMetastoreCatalog.tableExists(HiveMetastoreCatalog.scala:152)
at org.apache.spark.sql.hive.HiveContext$$anon$2.org$apache$spark$sql$catalyst$analysis$OverrideCatalog$$super$tableExists(HiveContext.scala:309)
at org.apache.spark.sql.catalyst.analysis.OverrideCatalog$class.tableExists(Catalog.scala:121)
at org.apache.spark.sql.hive.HiveContext$$anon$2.tableExists(HiveContext.scala:309)
at org.apache.spark.sql.hive.execution.CreateTableAsSelect.run(CreateTableAsSelect.scala:63)
at org.apache.spark.sql.execution.ExecutedCommand.sideEffectResult$lzycompute(commands.scala:53)

Author: wangfei <wangfei1@huawei.com>

Closes #4365 from scwf/ctas-exception and squashes the following commits:

c7c67bc [wangfei] no used imports
f54eb2a [wangfei] fix exception for CTAS