_write_partition: parted should be executed with sudo

In _write_partition parted is not currently executed with sudo:

--> execute('parted', '--script', dest, 'mklabel', 'msdos') <--
--> execute('parted', '--script', dest, 'mkpart', 'primary', <--
            '%ds' % primary_first,
            '%ds' % primary_last)

This can cause problems if nova-compute is not run as root, or if udev rules are not properly configured for allowing write access to the 'dest' device to the user that runs nova-compute.
Also, we found that even if udev rules are properly configured, parted still is unable to complete successfully.

It might be therefore better to run parted with sudo, while a more effective solution is investigated.