CBQE-1053: fix XDCRBaseTest._do_cleanup

http://qa.hq.northscale.net/job/centos-64-2.0-uniXDCR-biXDCR-sanity/397/consoleFull

Change-Id: Ib3a5aba2c9e46afaa24e2ac8826b74093018ae8c
Reviewed-on: http://review.couchbase.org/24789
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>