UTIL: Introduce subdomain_create_conf_path()
    
    This is a utility function that replaces the create_subdom_conf_path().
    Differently than the latter, it only takes one parameter and is going to
    be used in a few different places (thus adding it to util.h).
    
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Lukas Slebodnik <lslebodn@redhat.com>