Create the table directory automatically when querying create external table.

When creating an external table, if the table directory doesn't exist in the file system, tajo throws IOException.

Take this query for example:

default> create external table table1 (id int, name text) using text location 'hdfs://localhost:9010/user/tajo/test1';
ERROR: hdfs://localhost:9010/user/tajo/test1 does not exist

We need to allow the table directory automatically for above case because of user's convenience. FYI, hive allows this policy.