Replaces sudo with su in init script

resolves issue #800
http://stackoverflow.com/questions/8940771/switch-user-in-a-init-script
The script already uses start-stop deamon. Take a look at line 92 of the init script shipped with 0.8.4. where it runs `if which start-stop-daemon`
 If that package isn't installed, then it falls back onto the basic sudo (amazon, and most openstack images don't have start stop deamon installed by default, hence the reason my installation falls to start. 

```
        if which start-stop-daemon > /dev/null 2>&1; then
            nohup start-stop-daemon --chuid influxdb:influxdb -d / --start --quiet --oknodo --pidfile $pidfile --exec $daemon -- -pidfile $pidfile -config $config > /dev/null 2>&1 &
        elif set | egrep '^start_daemon' > /dev/null 2>&1; then
            start_daemon -u influxdb ${daemon}-daemon -pidfile $pidfile -config $config
        else
            sudo -u influxdb -g influxdb ${daemon}-daemon -pidfile $pidfile -config $config
        fi
```