CBQE: Rebalance task hangs if _rebalance_progress returns -100

it returns -100 when ServerUnavailableException

Change-Id: I7961ad8cdf54ec10bda829cbedb107a93c382a0b
Reviewed-on: http://review.couchbase.org/26188
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>