openstack: Use a service subnet for service-vm

[flaper87]

Move the service VM under its own subnet. This helps creating deployment-only resources that can be easily cleaned after the fact.

For example, we can set the nodes' subnet dns nameservers to point to the service-vm and then set it to something else after the deployment succeeds in scenarios where users bring their own DNS.

/cc @tomassedovic

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[tomassedovic]

Thanks! This looks great & we want it.

But I've noticed that the workers come up in the service subnet instead of the nodes one. Which means they still don't get the right DNS config.

It's not introducing any regressions, so we can do that in a follow-up patch. Let me know what you think.

[flaper87]

@tomassedovic probably better in a follow-up patch because I'm afraid that may not be related to this PR. :/

[tomassedovic]

/lgtm

[openshift-bot]

/retest

Please review the full test history for this PR and help us cut down flakes.