Add client for assisted volume snapshots API.

This will invoke post and delete calls on os-assisted-volume-snapshots
endpoint.

Closes-bug: #1668407

Co-Authored-By: Ghanshyam Mann <gmann@ghanshyammann.com>

Change-Id: I81e8fd9f89ec1cb00fc0116c31f9c0559406164
6 files changed