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 ...
- [ ] swift test should create a new container on every test run and delete it afterwards
I only wanted to have on config option to make it easier to set up an objectstorage based owncloud. It does not really make sense to have two separate configuration options like "root_storage" and "home_storage" which I removed with 547791b. But I need the configuration when setting up these storages, hence the IMHO more meaningful name of "objectstore". Does that make sense?