MB-100: skip SwapRebBase.common_tearDown when stop-on-failure for python2.6

Change-Id: I46a70acc740b24fbb09967111927b4bf00ee2252
Reviewed-on: http://review.couchbase.org/25255
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>