Add --save-ip option to allow the generated node config to be saved with the IP address instead of the server name.

The option --save-ip causes the generated node file to be saved as ip_address.json instead of :server_name.json. This option allows simpler scripting of 'knife solo cook' commands. This is useful for managing clusters without the use of Chef-Server.

These few lines are the first Ruby code I've ever written, if I've done something stupid/wrong/ugly please let me know.