Change generic-ip-address attribute from generic driver to generic-address

Testing the generic driver (https://github.com/docker/machine/blob/master/docs/index.md#generic) that is possible to use a domain name in the `generic-ip-address` attribute. 

For make easier to make others understand that is it possible, a good idea should be change it to just `generic-address`. I can make this PR if you guys like this idea. :+1: 
