ZEPPELIN-100 loading hive driver and then hive2 driver

[ZEPPELIN-100](https://zeppelin-project.atlassian.net/browse/ZEPPELIN-100)

This Pull request fixes hive jdbc driver loading order.
If hive2 jdbc driver is loaded before hive1 driver loads, then all connection is handled with hive2 driver implementation even if it's driver URL is jdbc:hive://.

If we load hive1 driver first and load hive2 driver, jdbc:hive:// is handled in hive1 driver and jdbc:hive2:// is handled in hive2 driver. this behavior is what we want.

So i moved driver class loading statement into init() method and load them in right order.