Update warning message ip address

[willingc]

In https://github.com/jupyterhub/configurable-http-proxy/blob/master/bin/configurable-http-proxy#L216, we caution against using '*' as an ip in the docs and in the warning a few lines above yet it is displayed by L216. Perhaps change to '0.0.0.0' or '' or emit a warning such a few lines above.

[minrk]

There's an ever-so-slight difference between '' and 0.0.0.0, where '' is all interfaces, while 0.0.0.0 is all IPv4 interfaces. This will make a difference approximately never, but we could use the text "all interfaces" when ip is blank.