Add validation for Storage Profile parameters

[CecileRobertMichon]

/kind feature

Describe the solution you'd like
[A clear and concise description of what you want to happen.]
As part of the AzureMachine webhook, we should validate any spec passed in as part of OSDisk to make sure that it is valid and that it won't cause an error when creating the VM storage profile.

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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[CecileRobertMichon]

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[srm09]

Working on this one.

[CecileRobertMichon]

/assign @srm09