Fix ip_addr -> ip_address in server delete

When I renamed this method for KNIFE-366 I neglected to check other files that I hadn't touched. It turns out that `rackspace server delete` uses it to display IP addresses as well. It'll fail with the rather cryptic "could not locate server" since `NoMethodError` is rescued.

Not sure if this needs another ticket or if you can just pull it in as part of KNIFE-366.