Allow concurrent tasks to complete after error

Add an option to allow concurrent tasks in a DependencyTaskGroup some time
to run to completion before being cancelled in the event of an error in
another task.

Change-Id: Ie3d63a0d5e8c9fa5f0fa74285b06f732685afcbc
Reimplement Volume snapshot with backup

Use backups instead of snapshots for volume handle_snapshot: it allows a
better management of a lifecycle of the stack snapshot, because cinder
snapshots can only exist along with the original volume.

blueprint stack-snapshot
Co-Authored-By: ala.rezmerita@cloudwatt.com

Change-Id: I1ec2bd177df61cae05769628c63db0bd0790c3ce
Merge "Allow concurrent tasks to complete after error"
Merge "Reimplement Volume snapshot with backup"