Attach ISO images to the separate cdrom device

* For each ISO image the individual cdrom device will be added to the VM. During the cleanup these devices will be deleted.
It makes attach steps more clear - there is no doubt what is the name of the device.

* CD/DVD drive with installation ISO should be exactly the 2nd device in the VM boot order.
This ensures that VM will boot from this ISO at the first boot time only.

P.s. @JessThrysoee, I send this pull-request to you, because when you merge these commits here, they automatically append  your changeset in the mitchellh/packer#1502