Make cf_local_ip() to use proxy's ip for hint if the machine is behind proxy

cf_local_ip() raises following exception if the machine is not reachable to the Internet.

`Errno::ENETUNREACH: Network is unreachable - connect(2)`

So, if the machine is using proxy to install vcap, cf_local_ip should use its proxy server's address as a hint to know its local address.

Please also note that cf_local_ip does not receive argument for hint ip anymore since I believe it is not very useful.