This PR is a part of the actions necessary to make Certbot-CI work on Windows, in order to execute the integration tests on this platform.
I initially used the fully-fledged HTTP proxy [Traefik](https://docs.traefik.io/) to distribute HTTP challenges among several pytest nodes, and so parallelize the integration tests. Traefik for this purpose is overkill. We just want to redirect the ACME server to a pytest node depending on the `Host` header, and we use here a production-grade HTTP proxy for that.
However it was not a problem on Linux, as soon as you can have Docker, because this instance is deployed through it.
But this becomes a problem for Windows, where Docker is not available everywhere, very compelling on its setup, and limited by the implemented network drivers. See my comments here https://github.com/letsencrypt/pebble/pull/240 for more details.
Hopefully Python ships with everything needed to implement a simple HTTP proxy, with strictly what we need for the parallelization of integration tests.
This PR implements this kind of HTTP proxy, and remove the coupling to Traefik.
This PR has been tested successfully with integration tests on Pebble under Linux for Python 2.x and Python 3.x, and the proxy alone has been also tested successfully on Windows (no integration tests can be run for now on this platform).
* Create a python proxy
* Refactor proxy config
* Working logic
* Resolve from the path
* Give proxy process to the ACMEServer context manager
