Replaces sudo with su in init script

resolves issue #800
http://stackoverflow.com/questions/8940771/switch-user-in-a-init-script
@damm yes that will work too, the only advantage of this syntax is it works elegantly if the influxdb users default shell is /bin/false

I'm open to using the other syntax, shouldn't it still specify the pid file though?

    su - influxdb -c "${daemon}-daemon -pidfile $pidfile -config $config"