Currently, it is impossible to assign existing floating ip to new droplet
with one api call since droplet creation api does not have that field.
Although that can be achieved by calling additional API (
https://developers.digitalocean.com/documentation/v2/#assign-a-floating-ip-to-a-droplet) after it's created, it would be nice if I can do with one api call.
Also it would solve a problem of "loosing ip address" - see reserve IP Addresses / Make sure we can keep an IP if we recreate a droplet.