Create one nodes instance group per zone

[johngmyers]

Creates one nodes instance group per zone, restricted to that zone. Changes the default number of nodes to the number of zones. If fewer nodes than zones, only creates one instance group per node.

Fixes #8152

[hakman]

If fewer nodes than zones, only creates one instance group per node.

I would go with changing what number of nodes means and applying it to each instance group.

[olemarkus]

I would also change the meaning of node-count to be number of nodes per AZ. Isn't that what happens as the implementation is exactly now?

[hakman]

@olemarkus at the moment an ASG can span multiple AZs so, you just set a fixed number of nodes that are randomly assigned to an AZ.

[olemarkus]

Sorry, I missed a change in new_cluster.go.

I would either change this to nodes per AZ or documenting that this is the total number of nodes in the cluster. If we go for nodecount remaining the total number of nodes in the cluster, how we distribute the nodes across zones should be mentioned in the docs too.

[johngmyers]

--master-count is the total number of master nodes, not the number of master nodes per IG. Making --nodes-count be the number of nodes per IG would be inconsistent.

The documentation says "Set number of nodes. Defaults to one node per zone", which is precisely the behavior implemented.

[hakman]

/lgtm

[hakman]

/retest

[rdrgmnzs]

Thanks @johngmyers !

/lgtm
/approve

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