POC for letstest redesign

[adferrand]

Some explanations of what is done here:

• the general concept of letstest is kept: launching several bash scripts on different OS environments to check that certbot-auto, certbot and the plugins are behaving correctly
• letstest here is a part of certbot_integration_tests, and so takes advantage of the existing logic of certbot-ci: an ACME server (pebble or boulder) is deployed on demand using Docker, and HTTP proxies + DNS are set up to allow parallel executions of tests on this ACME server
• bash scripts are in asserts/farm_tests: they will be executed on each target environment, and have access to several environment variables to be able to connect to the ACME server
• the target environments are Docker, specifically set up to mimic as close as possible complete environments: it consists essentially in having a full systemctl up and running, in order to make real modifications in the target environment using apache or nginx plugin
• the target environments are defined as Dockerfile in assets/farm_envs: it allows specific customization depending of the target OS (centos, fedora, ubuntu...), and they are built before the tests start
• pytest executes in parallel over the tests and the target environments, based on the content of assets/farm_tests and assets/farm_envs respectively: there is a tox target to test that: tox -e letstest
• all this logic is tied together using fixtures and parameterized tests functions in a pytest-style module, taking advantages of the same specificities than in other integration tests (reporting, output capturing, CLI params, and so on)