Support excluding certain directories from quota

[yuvipanda]

We have _shared and _shared-public directories that should have a different quota setup. While we wait for implementation of a tiered quota config system, this simply allows us to exclude them from being quota'd

[yuvipanda]

need to test that it's actually being excluded

[sgibson91]

We should also include allusers here. One way for a hub champion to self-serve the scenario "a user downloaded a big file and now their server won't start" would be to have the allusers directory mounted so they could delete $bigFile themselves.

ETA: I realise this is a PR adding a generalised flag. Just wanted to be explicit with my thinking and what we (2i2c) should consider setting as default dirs to exclude in basehub.

[sunu]

@yuvipanda @sgibson91 - I messed up this branch a bit while adding a test. Continuing in #19