pithos: Fix KeyError raised by the backend

If not an external quota component (eg astakos) is used the Pithos+ backend
enforces the quota restrictions.
However, if no quota limit is set for an account, it expects a default quota
policy to be set.
This commit fixes this issue.
pithos: Implement 'delete_by_uuid' backend method.

Extend the Pithos+ backend by providing a method that gets as input the
object's UUID and deletes the object.
pithos: Provide tests for 'delete_by_uuid' method.