[openstack] fix floating ip attaching

- Check that floating IP already attached;
- Fix floating IP attaching.

I really don't know how it work previously, cause in fog_driver.rb attach_ip() executes in different ways from attach_*() functions.
FYI, `perform_action` will print little green text saying this changed, and mark the resource as updated--I think you wanted to put the `perform_action` call inside attach_ip_from_pool, where you know for sure you are going to attach floating ips.