Add device_owner attribute to port

Add device_owner attribute to port

in order for firewall logic to be able to poke a hole allowing traffic to/from a DHCP server,
we need to expose the IP being used for DHCP in a subnet to external entities.
This commit adds device_owner attribute, then dhcp-agent will update the device_owner value
of port to dhcp-agent.

Implement blueprint expose-dhcp-server-ip
Change-Id: I11283485bff8a3a3cf0b1a2716763ad32e43028a