CBQE-0 :: fix graceful failover stop test case failure

The test case was failing since we stopped graceful failover. And before the nodes were healthy tried graceful failover again, which led to failure. Added sleep

Change-Id: I95843bcfded04f969f7d929708e33dfb5fc733f4
Reviewed-on: http://review.couchbase.org/40282
Reviewed-by: Thuan Nguyen <soccon@gmail.com>
Tested-by: buildbot <build@couchbase.com>