Extend `maxStartForTaskForQueue` to scope for specific user

In order to have more granular control over the task quotas this
chage introduces ability to specify the user for which we need
to limit concurrent running tasks.

Example Usage:
  [quota "*"]
    maxStartForTaskForUserForQueue = 20 uploadpack servicebot SSH-Interactive-Worker
    maxStartForTaskForUserForQueue = 10 uploadpack admin SSH-Batch-Worker

Change-Id: I2010a02bbdf7a3660089af667c8dce0a799d64b1