Support for preemptible nodes

[lantier]

Is it in the roadmap for support?
I think this can solve without minor issues:

node_config {
    preemptible    = "${lookup(var.node_pools[count.index], "preemptible", false)}"
}

[morgante]

Yeah, we can definitely add this. PR's welcome as well!

[czka]

Code changes should not be accepted without respective doc updates. Only accidentally I have learned that preemptible nodes are already supported in this module. I have almost wasted my time trying to add an already existing feature. Now I'm wasting it writing this comment.

[morgante]

@czka Point well taken. As an immediate measure I added it to the usage example (f057964). Longer term, we need to improve documentation of the node_pools variable (#45).