* nixos/k3s: simplify config expression
* nixos/k3s: add config assertions and trim unneeded bits
* nixos/k3s: add a test that k3s works; minor module improvements
This is a single-node test. Eventually we should also have a multi-node
test to verify the agent bit works, but that one's more involved.
* nixos/k3s: add option description
* nixos/k3s: add defaults for token/serveraddr
Now that the assertion enforces their presence, we dont' need to use the typesystem for it.
* nixos/k3s: remove unneeded sudo in test
* nixos/k3s: add to test list
