Support Storage Policy

* The file path on mountpoint was /container/obj and has been changed to
  /account/container/obj. This structure is not created on account and
  container creation but rather during the first object PUT to that
  container.
* Rename OnDiskManager class to DiskFileManager
* Remove overridden container_update() method and use Swift's default
  implementation that updates container DB.
* Add support for async_dir in storage policies - failed database
  updates are stored in these directories to be later picked up by
  object-updater daemon. This change reintroduced write_pickle and
  pickle_async_update methods which are slightly modified versions
  of Swift's methods of the same name.
* Updated tox.ini dependency on Swift to point to feature/ec branch of
  Swift's github repo.

Signed-off-by: Prashanth Pai <ppai@redhat.com>