netdev-tunnel: put remote_port into the DB

netdev-tunnel: put remote_port into the DB

We put the remote_port for each tunnel into the DB, so that it shows in
the output of 'ovs-vsctl show' and persists across local restarts.
Accordingly, a tunnel is now created thusly (we show only one side):

ovs-vsctl add-port br0 tun0 -- set interface tun0 type=tunnel
ovs-appctl netdev-tunnel/get-port tun0

Then, when the remote port is known:

ovs-vsctl set interface tun0 options:remote_ip=... \
	options:remote_port=...

(The remote_ip and the remote_port can also be set independently)