kops: restructure package to maintain multiple versions in the future

kops versions are bound to the version of the kubernetes cluster they
are targeted to maintain (kops 1.12 -> k8s 1.12, kops 1.13 -> k8s 1.13,
etc). Upgrading kops should therefore be done very deliberately
as it may affect the cluster it is working with in drastic ways.
This change introduces the ability to maintain multiple versions of
kops in nixpkgs, giving the users the ability to pin it to their target
cluster version when installing