Selectively disable TLS

[coopernetes]

After #558 , Git Proxy can no longer run without a private key & certificate available. Not all deployments are appropriate to run a direct TLS server such as when Git Proxy is deployed behind a shared load balancer or using a cloud SDN such as Envoy to encrypt app-to-app traffic at a host level.

I believe the intention behind that PR is still valid. Given how easy it is to generate a cert these days via LetsEncrypt, the project should provide HTTPS support for the app out of the box. However, it should be configurable to disable HTTPS if such a deployment is suitable.