Define validation strategy for provider config

[rsdcastro]

As per comment in #405, "validation is another concern and independent of specifying the cloud provider in the top level object or in the provider specific config. We can't bake provider specific validation logic into the API. That needs to be pluggable somehow through a webhook, admission controller, or some other mechanism. But it's a much larger discussion."

This issue tracks methods we'll allow for validation, including recommendations and examples.

cc @karan

[roberthbailey]

This seems to depend on #504

[rsdcastro]

This issue was moved to kubernetes-sigs/cluster-api#44