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.