Support Snapshot policy in volumes

This handles the Snapshot DeletionPolicy in the AWS::EC2::Volume
resource, creating a backup of the volume before deleting it. It
requires a recent cinder client with backup support.

Implements: blueprint volume-snapshots
Change-Id: Idbb14a434ad6ccde9f198e07799e8aef0867dd6f
Merge "Support Snapshot policy in volumes"