use standard_attr db from neutron-lib
use standard_attr db from neutron-lib

This patch consumes the neutron_lib.db.standard_attr module from
neutron-lib by switching all imports to use neutron-lib's standard_attr.

Change-Id: If1b5adb4069813df0e9c7597bf84103ebbae7242

openstack/networking-bgpvpn,openstack/networking-bgpvpn,openstack/networking-bgpvpn