CBQE-1031: teardown/setup fall in xdcr test if ServerAlreadyJoinedException

if in setup for xdcr tests ServerAlreadyJoinedException tearDwon never execute

Change-Id: Ieeb678a133b2b3910f2e8c24ef917b67d6d61729
Reviewed-on: http://review.couchbase.org/24473
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>