Fix empty local_ip in ml2_conf.ini on undercloud

empty local_ip in ml2_conf.ini would make neutron-openvswitch-agent
fail to start, then fail to bridge dhcp to br-ctlplane and pxe boot an
overcloud, so provide the value in undercloud-source.yaml.

Related-Bug: #1394956

Change-Id: If3a94b9c2b971ceb7601f91a2db64989960fb5d3