All the volume_tasks logic has been redistributed as methods of either
cinder or nova client plugins.
The logic of detaching was simplified and a check for out-of-band
changes is no longer executed.
The old VolumeAttachTask is left in place as it is still used in
creating the AWS::EC2::Instance resource.
Change-Id: I13f8e03ff090dd5f5739e5af231c77d066eb9eac
Partial-Bug: #1393268