Add support for notReadyAddresses

[Gsantomaggio]

Fixes #24

Add K8s StatefulSets example

[michaelklishin]

Is this a typo or we return an empty list instead of an address? Or did you mean

autocluster_log:info("No nodes ready yet!", [])

[Gsantomaggio]

It is not a typo
We return an empty list, since the POD is still starting. Most likely is itself.
here we are in this situation:

"subsets": [
    {
      "notReadyAddresses": [
        {
          "ip": "172.17.0.2",

Where we have only not ready nodes.

Here I had two ways to handle this:

1. Since the StatefulSets can start the POD sequentially, most likely is the first node.
2. Consider notReadyAddresses as possible nodes to join and trust on auto-delay startup to join this node.

[dcorbacho]

Log message is not strictly true, we might have several subsets: some have ready addresses and others not. If we want to notify that there are not ready addresses, we could do something like:

maybe_ready_address(Subset) ->
case proplists:get_value(<<"notReadyAddresses">>, Subset) of
      undefined -> ok;
      _ -> autocluster_log:info("k8s endpoint listing returned nodes not yet ready", [])
  end,
  case proplists:get_value(<<"addresses">>, Subset) of
      undefined -> [];
      Address -> Address
  end.

@michaelklishin any thoughts?

[michaelklishin]

This is a good idea and will simplify debugging.

[michaelklishin]

@Gsantomaggio this needs a manual merge into master.