MB-100: signal invalid number for long rebalance

- if rebalance cannot finish in the test scope, should post a 0
on dashboard

Change-Id: Ibe4260ed36c1013334ba82a32a742ed250d90374
Reviewed-on: http://review.couchbase.org/23828
Reviewed-by: Ronnie Sun <ronnie@couchbase.com>
Tested-by: Ronnie Sun <ronnie@couchbase.com>