nixos/haproxy: Revive the haproxy user and group

Running haproxy with "DynamicUser = true" doesn't really work, since
it prohibits specifying a TLS certificate bundle with limited
permissions. This revives the haproxy user and group, but makes them
dynamically allocated by NixOS, rather than statically allocated. It
also adds options to specify which user and group haproxy runs as.