:sparkles: Add support for autoscaling to/from 0

[nikParasyr]

What this PR does / why we need it: Add support for autoscaling to/from 0. To do this:

• Follow the contract for doing this as documented here
• Add a new controller to patch OpenStackMachineTemplate.Status accordingly
• Add the feature behind a feature flag

Which issue(s) this PR fixes: Fixes #1328

Special notes for your reviewer: I'm currently not adding support for:

1. Architecture info: Could not find a standardized flavor extra_spec for this one. There are some traits/resource specific ones but they seem to be for when ironic is used. Images can have an architecture attribute but im not sure whether this should be used => I'd prefer to leave this out of this PR and see if users need/require this.
2. GPU info: Again couldn't find the standardized extra_spec for this. I would also need to check what type of info autoscaler expects ( just vendor info or even gpu model info etc ) => I'd prefer to leave this out of this PR and see if users need/require this.

TODOs:

• [ ] squashed commits
• if necessary:
  • [x] includes documentation
  • [X] adds unit tests
  • [X] adds e2e tests

/hold

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[k8s-ci-robot]

Hi @nikParasyr. Thanks for your PR.

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[bnallapeta]

/ok-to-test

[k8s-ci-robot]

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[nikParasyr]

/test pull-cluster-api-provider-openstack-e2e-test

[k8s-ci-robot]

@nikParasyr: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-cluster-api-provider-openstack-e2e-test	0986a373625935ddcbc116eb3120159d62167626	link	true	/test pull-cluster-api-provider-openstack-e2e-test

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository. I understand the commands that are listed here.