pithos: Enable stale snapshot better handling

Extend Pithos backend functionality in order to
be able to set the state of the snapshot.
This commit introduces the following modifications:
* Changes 'available' flag (denoting the existence of the object mapfile
  in the backend) to integer.
* Expects for the 'available' field only the following values:
  -1 (MAP_ERROR), 0 (MAP_UNAVAILABLE) and 1 (MAP_AVAILABLE)
* Provides a respective database migration script.
* Adds 'update_object_status' method in PithosBackend that is used in
  order to update the state of a Pithos object.
* Returns 400 (BAD REQUEST) response status when one tries to download a
  broken snapshot.
pithos: Test update object status functionality
docs: Document weblogin