Interface with 0 DPN ID is created in ELan model

When OVS node is added after a provider network is already configured, the node's external interface is added with 0 as the DPN ID in the elan interfaces model and in the ietf-interface model.

In elan-interfaces
{
"name": "0:eth1:flat",
"elan-instance-name": "912ea4a4-cf0c-4f31-a08c-193a2442c826"
}

In ietf-interfaces
{
"name": "0:eth1:flat",
"enabled": true,
"odl-interface:l2vlan-mode": "transparent",
"type": "iana-if-type:l2vlan",
"odl-interface:parent-interface": "0:eth1",
"odl-interface:external": true
}