Accept Enet route without IP address from agent

Agent can now advertise a MAC without IP for L2-only VNs.
Or it can set the IP to 0.0.0.0/32.
Control Node advertises with IP as 0.0.0.0/32.
Control Node builds XMPP id using 0.0.0.0/32.

Change-Id: I3e52337e1722da679023fef5fafd2f816cb33509
Partial-Bug: #1366409