CBQE-0: rabbit_helper allow vhost arg

we also need to support passing the --cluster
value used in cbsystest to automate other workers

Change-Id: I2acdbdb82730c90f532bd875f004ca2496203540
Reviewed-on: http://review.couchbase.org/25644
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Chisheng Hong <chisheng@couchbase.com>