Parse HAProxy files to get a mapping of cert paths to sockets

It would be easier for OPS if we could parse (multiple) HAProxy config files, those point to both the cert paths, multiple are possible and the socket they belong to (only one possible) so it will lead to path/socket pairs eliminating the need to specify them in arguments or config in order. Since HAProxy is the main use case for stapled, it makes sense to make this one of the generic features.

1. So the suggestion is to add parsing of HAProxy config files (only a few directives, regex should do).
2. Add a CLI argument --haproxy-config or similar.