nova-manage cleanup up dangling volume attachment

It is easy to create dangling volume attachments where nova thinks a volume is attached to an instance while the volume is already deleted in cinder by first deleting the attachment in cinder then deleting the volume in cinder. [1] This makes the instance state inconsistent.

We could extend the nova-manage volume_attachment CLI to be able to
1) list dangling attachments in a deployment
2) clean up such attachments by deleting them from the nova database.

To be on the safe side such cleanup should only be executed if the VM is in shutoff or shelve offloaded state.

[1] https://paste.opendev.org/show/bb5oX3pM4Vmw9RJCRa0v/