This is useful for, e.g. OCSP stapling.

Switch to certbot pem names

See merge request !1

Easier to swap between the two.

Support multiple config_dirs, name all paths explicitly instead of
simply using predefined subdirectories.

Add dns-01 support, make the code more readable, add a testing
mode that will generate self-signed certificates (for test
environments that are not reachable from outside).