builder/parallels: Do not delete the first cdrom device

It is good practice for Vagrant boxes (and preconfigured VM images as well) to have at least one virtual cdrom device enabled. Even some guest OS types (for example, boot2docker) required this device.

But now Parallels builders deletes all cdrom devices while the `step_remove_devices`. 
I've modified the command: `cdrom0` device will be just set to 'Default CD/DVD' which is emulated on all hosts by Parallels Desktop.

cc: @rickard-von-essen, @yungsang