Fix unsubscriptable-object error
Fix unsubscriptable-object error

After astroid upgrade from 2.3.3 to 2.4.0, we hit
unsubscriptable-object error:
neutron_vpnaas/db/vpn/vpn_db.py:197:
E1136: 'ipsec_site_conn_db' is unsubscriptable
neutron_vpnaas/db/vpn/vpn_db.py:622:
E1136: Value 'endpoint_group_db' is unsubscriptable

Use db attribute while vpn_models not define _getitem__()
for getting object values.

Change-Id: If0b83c693eedba5eb11dcda7abe07f51e4539516

openstack/neutron-vpnaas,openstack/neutron-vpnaas