Allow user to customize letsencrypt server to contact

[clems4ever]

There are several use cases in which users would like to use a internal server. For example, it is useful if one wants to handle internal certificate issuance within his cluster with an internal letsencrypt boulder. It is also useful for testing a cluster setup without contacting the public letsencrypt service.

[mattj-io]

This has been added to the latest version, and the main repo for this project has moved to https://github.com/dcos-labs/letsencrypt-dcos