add az_list_node variable to specify different AZs for kubelets

[rptaylor]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespaces from that line:

/kind api-change
/kind bug
/kind cleanup
/kind design
/kind documentation
/kind failing-test
/kind feature
/kind flake

What this PR does / why we need it:
Since kubelet nodes can be considered ephemeral/disposable , different availability considerations may apply than for master nodes. Accordingly, on Openstack users may wish to use different AZs or different balancing across AZs for kubelet nodes than for master nodes. This PR adds a TF variable az_list_node which is exactly like az_list, but it applies only for the k8s node type. The default value is the same, so there is no change in behaviour unless users choose to modify the variable.
This provides finer control over placement of different types of nodes on Openstack.

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

No

[rptaylor]

/assign @holmsten

[rptaylor]

Also fixes some deprecation warnings for TF 0.12

[huxcrux]

I would say that this is a user facing change? Most Openstack installation have custom AZs set meaning this might break clusters for users that doesn't read the code.

This should probably be added to the Openstack terraform documentation as well (https://github.com/kubernetes-sigs/kubespray/blob/master/contrib/terraform/openstack/README.md)

[Miouge1]

Can we make this backwards compatible? Something like if az_list_node is not set then default to az_list ?

[rptaylor]

@Miouge1 @bl0m1
Okay, so az_list_node could be an empty list by default.

Then there could be a conditional (ternary) expression, something like

length(az_list_node) > 0 ? az_list_node : az_list

Would that be a good way?

[huxcrux]

Looks good :)
/assign @holmsten

[holmsten]

/lgtm
/approve

[k8s-ci-robot]

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