Add Ubuntu 14.04 Jenkins Docker slave

Change-Id: I4e23c6f8a415201f177bb9ade380af1d8e9fccac
Reviewed-on: http://review.couchbase.org/44638
Reviewed-by: Chris Hillery <ceej@couchbase.com>
Tested-by: Chris Hillery <ceej@couchbase.com>