Deployment with controllers in non-default nodegroup cause problems with floating IPs

I suppose the best way to fix it is to add validation to pre-deployment nailgun checks, it should check that provided floating IP range matches public network of node group with controller(s). If there's no controllers in environment, then no additional validation is needed. Assigned to fuel-python.

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Actually, we should not hardcode such limitations to 'controller' roles because it will make it impossible to detach network nodes from controllers. So the best way is to describe this in the documentation in the section provided by Maksym. Assigned to docs team.