Fix sporadic failure in failover log test on centos

We need to make sure items are persisted before we do a force
shutdown on the engine in order to guarentee that the stats we
check later will be correct.

Change-Id: I44fd06eb6c129d995b10afbeeab1ef2b96e6f46e
Reviewed-on: http://review.couchbase.org/33315
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>