Parallels: Fixed issue #1667

1) Fixed issue #1667 by partially reverting the previous changes.

While attaching the bootable ISO to the DVD/CD-ROM device we will attach it exactly to the first one, `cdrom0`. 
It is caused by Parallels Desktop 9 bug - virtual machine is failed to boot from any cdrom device except the first one (regardless of its enabled/disabled state). 

So, on the cleanup stage we will detach the ISO with a little trick, by setting the image path as an empty string.
It is working fine on both of supported versions: Parallels Desktop 9 and 10 as well.

2) Setting the boot order moved to the separated step, `stepSetBootOrder `.  It makes the build process more clear.
P.s. This setting is still required because HDD is added (by `stepCreateDisk `) to the end of boot order.