CBQE-0: if test_failed & stop-on-failure=True then skip cleanup in XDCR_BT

Change-Id: Idb1abc13140ad24b1b67f7a6adb7a62a2a713634
Reviewed-on: http://review.couchbase.org/38398
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>