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

[bharatnc]

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