L3 DB: only read dns 'dns-integration' once
L3 DB: only read dns 'dns-integration' once

Check only once that the extension is supported. This does not
need to be done every time a floating ip is created or updated.

TrivialFix

Change-Id: I3fac31a531c9f6b556a954f5a2d264cc64758f3b

eayunstack/neutron,sebrandon1/neutron,igor-toga/local-snat,cloudbase/neutron,mahak/neutron,eayunstack/neutron,noironetworks/neutron,openstack/neutron,huntxu/neutron,openstack/neutron,igor-toga/local-snat,mahak/neutron,huntxu/neutron,noironetworks/neutron,mahak/neutron,cloudbase/neutron,sebrandon1/neutron,openstack/neutron