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 might want to cache/buffer such calls later to reduce the number of API calls as I did here: https://github.com/owncloud/core/pull/7897

To be done separately as part of performance improvements