wait for RunTime only

in gateload sleep time is calculated as:

    time.Sleep(time.Duration(config.RunTimeMs-config.RampUpIntervalMs) * time.Millisecond)

Change-Id: I7e522aba001021d5c7fce1d8292ac6b1ec3204ba
Reviewed-on: http://review.couchbase.org/36938
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>