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
We had issues in the past with recursive deletion when having too many subdirs. (CC @DeepDiver1975)
Maybe better to replace it with a flat deletion.

To be done later in a separate PR.