Ignore dead subscriptions in Swift backend
Ignore dead subscriptions in Swift backend

When listing subscriptions, the ones that are expired still appear in
the container, but retrieving the object will return a 404. We need to
ignore that 404 to indicate that the subscription is gone.

Change-Id: Ic66827f7bac701299b565e1a49ba5008718503c5
Closes-Bug: #1693862

openstack/zaqar,openstack/zaqar,openstack/zaqar,openstack/zaqar