✨ Add MachinePool support for FailureDomains

[fiunchinho]

What this PR does / why we need it:

We need a way of knowing which FailureDomains to use when creating a MachinePool.

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 #

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[k8s-ci-robot]

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[vincepri]

The change looks like it's only adding the type, but it isn't used anywhere yet?

/assign @juan-lee @CecileRobertMichon

[fiunchinho]

The change looks like it's only adding the type, but it isn't used anywhere yet?

/assign @juan-lee @CecileRobertMichon

I plan to use it here.

[vincepri]

/ok-to-test

[detiber]

It might be good to update the MachinePool proposal to include how this field is expected to be consumed by provider implementations: https://github.com/kubernetes-sigs/cluster-api/blob/691f2688ec648f5940bf443fd64939fd53b1aa57/docs/proposals/20190919-machinepool-api.md

[fiunchinho]

Updated design proposal.

[vincepri]

/lgtm
/milestone v0.3.7

/assign @detiber
for final approval

[vincepri]

PR needs squash, otherwise seems ready to go

[fiunchinho]

squashed

[k8s-ci-robot]

@fiunchinho: The following test failed, say /retest to rerun all failed tests:

Test name	Commit	Details	Rerun command
pull-cluster-api-e2e	6e6ba84	link	/test pull-cluster-api-e2e

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.

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[fiunchinho]

/retest

[detiber]

/lgtm

[vincepri]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: fiunchinho, vincepri

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