CBQE-1615: add infrustructure for dgm tests

Change-Id: I1c420d951efef0acc68d460d5903904580f5675e
example: -t  rebalance.rebalancein.RebalanceInTests.rebalance_in_with_ops,nodes_in=2,standard_buckets=1,items=10,active_resident_threshold=99,dgm_run=True
Reviewed-on: http://review.couchbase.org/30254
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>