Only display networks for which a node has an IP address

On the show node page, we were displaying all networks known to a node,
including the ones that don't have any IP address associated to any
interface. This is kind of useless (if there's no BMC, for instance).

Instead, we only display networks for which there's an IP address.
Show DHCP IP address when node is being discovered

When ta node is being discovered, it's using a temporary DHCP address,
and not the IP addresses defined with the network cookbook.

It's useful for people to know the temporary IP address so they can log
in the sledgehammer image if needed.
Merge pull request #744 from vuntz/only-display-relevant-ips

Only display relevant IP addresses on the node page
+1 tested