Our current approach to enforce the disk limit per job can be very
expensive by running 'du' in a loop. When having many repos in the
cache and many running jobs this can poison the cache and induce a
large amount of IO load. This can influence overall performance
especially if zuul is running on a shared storage like ceph.
Change-Id: Ic03168e30e0cba4a4adb42eebf4709ceba0d8c3e