MB-100: skip Basetestcase.teardown when stop-on-failure for python2.6

Change-Id: If0219053315352daada8d5fe2934624e009cfd65
Reviewed-on: http://review.couchbase.org/25019
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>