Fix ipam_driver config help
Fix ipam_driver config help

Commit I1d633810bd16f1bec7bbca57522e9ad3f7745ea2 changed the default ipam
driver from none to 'internal', but the config help was not updated.

Change-Id: Ied714bf23bcedfface757de1f93827e30cbde59e

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