MB-7168: sleep 30 sec after reb falied when killed memcached

Change-Id: I22d271701848fe3c6cd05466afa7af3b47861f35
Reviewed-on: http://review.couchbase.org/26185
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>