Multiple driver loading capabilities

Zeppelin driver implementation is pluggable. However, it support single driver only at a time.
If Zeppelin can load multiple driver at a same time and can use it, it'll be really convenient.

Detailed proposal is in [ZEPPELIN-78](https://zeppelin-project.atlassian.net/browse/ZEPPELIN-78)

This Pull request include NO implementation yet, so should NOT be merged.
Creating this PR to share code work during progress.