Update ubuntu-1004 Dockerfile to full-fledged buildbot slave, include s3cmd

Change-Id: I17e82786db74d97f7bb5a167334885f73547a4d2
Reviewed-on: http://review.couchbase.org/39842
Reviewed-by: Chris Hillery <ceej@couchbase.com>
Tested-by: Chris Hillery <ceej@couchbase.com>