Added step to configure number of disc drives

[dongyuzheng]

I had a use case for this. Other people might as well.

Current behavior for ISO is to have 0 disk drives on the exported image
Exported XVAs to have the same number of disk drives as the original XVA

I think we should have consistent behavior:

Now, the default behavior is to have 0 disk drives
The user can specify the number of disk drives in the exported image with disk_drives config

When merging with #65
Keep this order:

        ...
        new(xscommon.StepConfigureDiscDrives),
        new(xscommon.StepConvertToTemplate),
        new(xscommon.StepExport),