MB-7981: Minimize sleep time for workloadgen

We have logic to handle topology change during workloadgen running.
However, the default cap time for sleep time during the rebalance
period will be 10 sec, which is proper for restore, but too much
for workloadgen because we do want to generate traffic during the
topology change period.

Change-Id: I8cbc6c493bf922cf73524a69143daafca6d466b3
Reviewed-on: http://review.couchbase.org/39214
Reviewed-by: Bin Cui <bin.cui@gmail.com>
Tested-by: Bin Cui <bin.cui@gmail.com>