Support ARM endpoint with non-trusted certificate

[honcao]

Is this a request for help?:
No

Is this an ISSUE or FEATURE REQUEST? (choose one):
Feature Request

What version of aks-engine?:
the latest version

Customer will provide the certificate to binding with external endpoint. e.g ARM endpoint.

The certificate could be non-trusted on the Linux Machine since customer could use cooperated signed certificate to deploy azure stack.

Right now, the k8s controller will be not be able to communicate back to ARM endpoint with non-trusted certificate.

AKS engine would need trusted the certificate and mount the certificate to the k8s controllers container.