Change stepback tasks to have a configurable priority above mainline commits

Right now, tasks started as part of the stepback process have the same priority than tasks started on the waterfall through manual scheduling and github requester. 

The consequence is that waterfall investigations take longer, leading to a longer task failure lifespan. The sooner we can complete the stepback, the sooner we keep the waterfall green (and subsequently folks patches).

This ticket is to increase the priority of stepback tasks above other mainline tasks so that the stepback process takes a shorter amount of time to finish.