We have implemented cascade deleting volume in CLI now,
but there's still something that should be improved:
1. A parsing problem of cascade is existing that always
evaluate parameter to True.
So now change the cascade parameter to a flag that don't
accept value anymore.
If user want to delete volume with snapshot, just need to
use 'cinder delete --cascade [volume_id]'.
Change-Id: I92e9400d5d7fbb741607bdbde7ac0c0667fca85d
Closes-Bug: #1585441
Implements: blueprint support-deleting-vols-with-snps-in-cli
86fe5617aa