CBQE-O: minor fix in FailoverBaseTest.tearDown

Change-Id: I202e9e2703ae046547efadf2b2b9ac3111a5bc92
Reviewed-on: http://review.couchbase.org/35268
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>