Cluster-api-provider-azure: Add support for managed identity with AzureMachinePool

[CecileRobertMichon]

/kind feature

Describe the solution you'd like
[A clear and concise description of what you want to happen.]
https://github.com/kubernetes-sigs/cluster-api-provider-azure/issues/312 and https://github.com/kubernetes-sigs/cluster-api-provider-azure/pull/616 added support for Azure Managed identities for AzureMachines. We should port the changes to AzureMachinePool to allow VMSS using MSI.

This will become especially useful if/when we add support for VMSS control planes.

Anything else you would like to add:
[Miscellaneous information that will assist in solving the issue.]

Environment:

• cluster-api-provider-azure version:
• Kubernetes version: (use kubectl version):
• OS (e.g. from /etc/os-release):

[CecileRobertMichon]

/help

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[shysank]

@CecileRobertMichon Is someone working on this? If not, can I take this one?

[CecileRobertMichon]

@shysank nope, go for it! You can comment /assign to assign yourself.

Let @nader-ziada or I know if you need any guidance.

[shysank]

/assign