CBQE-1220 fix the bug for choosing reboot or kill memcached for warmup

Change-Id: Ic97d23ca9c47b21223a1b4f00b45a6f90b668da2
Reviewed-on: http://review.couchbase.org/26740
Reviewed-by: Chisheng Hong <chisheng@couchbase.com>
Tested-by: Chisheng Hong <chisheng@couchbase.com>