Add kola test for large Ignition configs

[bgilbert]

Add a test verifying that Ignition accepts an arbitrarily large config, e.g. several megabytes. Certain platforms limit the size of userdata; in those cases we should test a config of exactly the maximum size. (That implies setting NoSSHKeyInUserData on those platforms.)

[prestist]

We re-evaluated this and found it still relevant.