prepare to support jackrabbit oak

* oak only has a single workspace. the test suite should run in that case too
* crx has the default workspace called different from 'default'

this is still missing a solution for the fact that we can not purge the repo with crx, as users are stored in the repo and we destroy the repo by deleting the admin user.

this depends on https://github.com/jackalope/jackalope/pull/165