Investigate using an alias for count and storage size in object store resources

[Brian Curtin]

Per comment on https://review.openstack.org/#/c/136926/1/openstack/object_store/v1/container.py, we should look into making use of the property alias feature when accessing counts and usage size - they come both in GET body and from HEAD, but with different values. We can probably unify them by using one property with an alias.