Object storage

don't merge before https://github.com/owncloud/3rdparty/pull/99 is in 3rdparty
- [ ] https://github.com/owncloud/3rdparty/pull/99 moves rackspace to 3rdparty, but files_external also provides it, causing tests to fail ...
- [x] swift test should create a new container on every test run and delete it afterwards
What's the reasoning behind not just returning an empty string here?