openssl cert integration example

[ZackButcher]

One piece of feedback we got from early users is that a simple example of CA integrations using openssl locally on a machine would be useful for understanding what "magic" the tool is doing and would help folks new to this to understand what's going on. I agree; I think we should add support for an openssl provider that just defers out to the openssl command on the local system to generate certs and load them into Istio. Maybe link to the page on istio.io that walks through using openssl to create the CA certs then contrast that with running istioctl gen-ca --config-file openssl.yaml