Add support for Spot VMs

[JoelSpeed]

What this PR does / why we need it:

Adds a spotVMOptions field to the AzureMachineSpec to allow users to request a Spot VM rather than a standard on-demand VM

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #407 (Should 407 cover machinepool too?)

Special notes for your reviewer:

Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

Release note:

Add support for Spot VMs to AzureMachine

[CecileRobertMichon]

@JoelSpeed do you have plans to document this feature in the cluster API book or does it make sense to add a short doc in here for now?

also FYI @devigned this is probably something we'll want to bring to the vmss implementation after both PRs merge or possibly in this one if #483 merges first.

[CecileRobertMichon]

Opened #575 to track AzureMachinePool support separately.

[JoelSpeed]

@CecileRobertMichon I was planning to have docs in the main CAPI book eventually, but not sure when that will be done. For now I'm happy to add a short doc in this PR which can be removed later once the full CAPI docs are done

[CecileRobertMichon]

@JoelSpeed this still needs docs + a rebase when you get a chance

[JoelSpeed]

@CecileRobertMichon Apologies for the delay! I have rebased and added a small doc, not sure how much is appropriate to write here, I can expand more if you would like

[JoelSpeed]

@CecileRobertMichon Is there anything more you want me to do with this? Did we want to raise the question about the spotVMOptions: {} emptiness being the default usage? I don't mind it but if you think there's a potential to improve the UX then we should discuss

[CecileRobertMichon]

@JoelSpeed the only two things left for me are 1) merge kubernetes-sigs/cluster-api#3063 and 2) resolve the question on spotVMOptions: {} usage. I'm not a big fan of having an empty "options" field to enable it but I've asked for second opinions on the thread above since this will likely affect CAPA as well.

[nader-ziada]

this looks good to me and ready to be merged, but asking @CecileRobertMichon if she has other comments since she has more context

[CecileRobertMichon]

/lgtm
/approve

Thanks @JoelSpeed 🎉

[k8s-ci-robot]

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