Create ACLs to allow different priority levels per groups

Actual:
- Only bots and admins can use high priority levels (<100)
- Swarming assumes cooperating clients that won't too silly things.

Goal:
- Create different priority level ACL based on the task request.

Risks:
- ACL maintenance a burden.
- Bots are running a task on behalf of an automated task running through a 
recipe. There's a lot of places where the tags and priority level could be 
changed, so trying to "lock it down" could be a worthless effort.

Original issue reported on code.google.com by maruel@chromium.org on 18 Dec 2014 at 2:47

madecoste / swarming

Create ACLs to allow different priority levels per groups #196