Implement admission controller for validation and defaulting

[mrIncompetent]

[p0lyn0mial]

related issue #224

[p0lyn0mial]

this admission controller should be implemented as a webhook.
See https://kubernetes.io/docs/reference/access-authn-authz/extensible-admission-controllers/#what-are-admission-webhooks

the API server that actually accepts/stores machines resources will make use of the webhook

[mrIncompetent]

@alvaroaleman please provide your example code

[alvaroaleman]

Here is an example mutating admission webhook: https://github.com/alvaroaleman/k8s-auth-injector