request-cert should have proper go modules files for managing dependencies

This project (request-cert) and it's dependencies aren't managed properly with a proper Go dependency manager (example, dep, glide go mod etc).

As `go mod` is becoming the standard these days, vendor all dependencies using go modules and commit the `go.mod` and `go.sum` files to the repo.

This will ensure that the project dependencies are tracked properly with every incremental change.

https://github.com/cockroachdb/k8s/tree/master/request-cert

cc: @joshimhoff