External Data Source: Frontend and catalog changes

Initial frontend and catalog changes for external data sources.

Change-Id: Ia0e61ef97cfd7a4e138ef555c17f2e45bbf08c18
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2224
Reviewed-by: Matthew Jacobs <mj@cloudera.com>
Tested-by: jenkins