Cache quota configuration fixes

This fixes a quota error when the cas directory does not exist yet and the quota is configured as percentage of total disk space.

This also partially reactivates test_invalid_cache_quota. That test was marked as xfail as part of the migration to buildbox-casd. This reactivates the test but removes the checks for too large quotas as that aspect is better handled by buildbox-casd. See BuildGrid/buildbox/buildbox-casd#28 (closed) for my suggestion how to handle low disk space scenarios.