DB base plugin: correct typo id to subnet_id.
DB base plugin: correct typo id to subnet_id.

Code is referring to not declared variable 'id'. It was working before
because the 'id' was defined in place where
_subnet_check_ip_allocations_internal_router_ports() was invoked.

Closes-bug: #1588281
Change-Id: I2d9331171bf59e5e375e0c4fd0eeec25f4ebff30

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