add password support to redis_constructor_info

a quick&dirty patch to add password support.



FYI, you are currently using /etc/munin/plugin-conf.d improperly leading to the following error:

    Parse error: Clutter before section start. at /usr/share/perl5/vendor_perl/Munin/Node/Service.pm line 110 at /etc/munin/plugin-conf.d/redis.conf line 1.
    Skipping the rest of the file at /usr/share/perl5/vendor_perl/Munin/Node/Service.pm line 110

instead of dropping a YAML file in there, munin uses it's own key/value system to inject ENVIRONMENT variables. plugins are then to extract their settings from ENVIRONMENT.

I will try writing a patch for that issue later.