db: Migrate "group snapshot" APIs to enginefacade
db: Migrate "group snapshot" APIs to enginefacade

Migrate group snapshot-related APIs from the legacy enginefacade to the
modern context-based enginefacade.

Change-Id: Ib6ca4372e005b4e35f49b4a8073a3d82661ab468
Signed-off-by: Stephen Finucane <4725dc7f3632bf126d9fbdc30c347e91e8d2fe16@redhat.com>

openstack/cinder,mahak/cinder,openstack/cinder,mahak/cinder