Unit tests don't need assets

This has driven me crazy for the last 8 months or so: unit tests shouldn't need to static assets.  The only reason we require this now is that some of our tests render Django templates, which use a static files tag, which raise a `ValueError` if the assets aren't found.

This PR fixes the issue by:

* Disabling the Django pipeline
* Using `NonPackagingPipelineStorage`, so that the Django template tag won't raise an error.
* For Studio, using `STATIC_URL` without the git commit (which otherwise causes some of the `static_replace` tests in common to fail).

This speeds up the unit test suite by about 48 seconds in Jenkins and is *significantly* faster in devstack (since it avoids copying to an NFS-synced directory)

I've preserved the settings for the acceptance tests, since those should more closely match production.

@jzoldak @benpatterson please review.