discovery: Include datapath name in log messages.

discovery: Include datapath name in log messages.

This commit adds the datapath name to discovery and DHCP-related messages,
so that it is obvious to the user where discovery is taking place.

Previously, messages looked like:
    Jun 04 13:41:29|00010|dhcp_client|INFO|sending DHCPDISCOVER

With this commit, they look like this:
    Jun 04 13:41:29|00010|dhcp_client|INFO|br0: sending DHCPDISCOVER

I may be the only person in the world who regularly uses controller
discovery.