waiters: Add wait_for_volume_attachment_remove

This change adds a specific waiter for tracking the removal of
attachments from a volume. This should allow callers to track the
detaching of multiattached volumes from instances while the volume
remains in an `in-use` state.

Related-bug: #1858841
Change-Id: I85917f683d2bc81fdf3f6d3351bf56fdb649740e
2 files changed