base: add check_disk check exception for Varnish

Varnish caches are purposefully at 99% full, since we fill the
partitions up with cache data. This makes check_disk unhappy, which in
turn makes us unhappy by having a huge list of cp* boxes CRITICAL (it'll
also make us ignore any real disk full checks for their /).

Add a gross hack with /srv/sd[a-b][0-3] as an exception and hope there
isn't any other class of boxes that matches this regexp.

Change-Id: I72a0b98f1bd5c4521ecef281fd2f0be8f52f8f3c