lower/stop raid initialization during installimage

the initial sync will start directly after creating the image. This will heavily slow down further operations like mkfs. We should slow it down like:

``` bash
echo 200 > /proc/sys/dev/raid/speed_limit_min
echo 300 > /proc/sys/dev/raid/speed_limit_max
```

or even stop the initialization. We've to ensure that the initialization will start after the boot.
