CBQE-0: fix xdcr warm up tests to warm-up dest_master if destination is given in the test parameter

Change-Id: Ib57f401bde8e0df9183fd6dc9f20877bdcca6150
Reviewed-on: http://review.couchbase.org/33467
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Sangharsh Agarwal <sangharsh.agarwal@globallogic.com>
Tested-by: Sangharsh Agarwal <sangharsh.agarwal@globallogic.com>