CBQE-0: clear logs in fail_over and add_back_node

Change-Id: Id3df4a42da31593267a363e43f421fdaea58c59a
Reviewed-on: http://review.couchbase.org/33786
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>