CBQE-1324: optimize autofailover code

all common verifications can be performed in ~wait_for_failover_or_assert

Change-Id: I28ea3e983b1733ee486f0f1fa59b259c7565c27b
Reviewed-on: http://review.couchbase.org/26673
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>