synchronize lookup and assignment of floating ip

Test kitchen can be run in parallel mode (-p option). When the driver is configured with floating_ip_pool option, it might try to assign the same floating ip address to two or more instances (and that might cause for two different tests to run on the same instance at the same time). 
The problem here is really a combination of issues. First is that the attach_ip_from_pool when executed at the same time can return the same free_addrs (and so that two threads will end up trying to associate the same address). Second, is that the server.associate_address address doesn't really return an error if the association fails. And, third, as far as I know the only way to verify the address assignment is to poll (e.g. by checking compute.addresses).

Since polling can be expensive and take some time or until somebody can suggest a better approach, for now I propose to synchronize the attach_ip_from_pool() method. That should at least ensure that the driver will not attempt to assign the same address twice. Obviously, this is not a foolproof solution as some other process might steal the address as the test kitchen runs but it makes the driver work a little better.