use correct manager for volume snapshots
use correct manager for volume snapshots

the snapshot manager is volume_snapshots, not snapshots.

Closes-Bug: #1567895
Change-Id: I12c0238fc04507c97e92089fda3bec816171a973

redhat-openstack/python-openstackclient,dtroyer/python-openstackclient,dtroyer/python-openstackclient,openstack/python-openstackclient,openstack/python-openstackclient,redhat-openstack/python-openstackclient