Define AGENT_TYPE_NEC in networking-nec repo
Define AGENT_TYPE_NEC in networking-nec repo

AGENT_TYPE_NEC is only used by networking-nec. That's why this change
defines it in order to allow its removal from neutron code.

Related-Bug: #1487598
Change-Id: I955fa48ee2120900e422bab57db250303c3d7bb4

openstack/networking-nec,stackforge/networking-nec,openstack/networking-nec,stackforge/networking-nec