skip adding floppy again in vmx_data_post step

With the new `vmx_data_post` of #1149 / #1152 I have the problem that the floppy is not unmounted properly. 
Watching the progress with PACKER_LOG showed me more details:

```
2014/06/08 23:03:14 ui:     vmware-iso: Unmounting floppy from VMX...
2014/06/08 23:03:14 /Users/stefan/go/bin/packer-builder-vmware-iso: 2014/06/08 23:03:14 Deleting key: floppy0.filename
2014/06/08 23:03:14 /Users/stefan/go/bin/packer-builder-vmware-iso: 2014/06/08 23:03:14 Deleting key: floppy0.filetype
2014/06/08 23:03:14 /Users/stefan/go/bin/packer-builder-vmware-iso: 2014/06/08 23:03:14 Deleting key: floppy0.clientdevice
2014/06/08 23:03:14 /Users/stefan/go/bin/packer-builder-vmware-iso: 2014/06/08 23:03:14 Deleting key: floppy0.present
    vmware-iso: Detaching ISO from CD-ROM device...
2014/06/08 23:03:14 ui:     vmware-iso: Detaching ISO from CD-ROM device...
2014/06/08 23:03:14 /Users/stefan/go/bin/packer-builder-vmware-iso: 2014/06/08 23:03:14 Writing VMX to: output-vmware-iso/ubuntu1204.vmx
2014/06/08 23:03:14 /Users/stefan/go/bin/packer-builder-vmware-iso: 2014/06/08 23:03:14 Floppy path present, setting in VMX
2014/06/08 23:03:14 /Users/stefan/go/bin/packer-builder-vmware-iso: 2014/06/08 23:03:14 Writing VMX to: output-vmware-iso/ubuntu1204.vmx
```

First the floppy is unmounted, but afterwards added again to the vmx file.

My PR skips this floppy mount in the StepConfigureVMX for the VMXDataPost config.
The final vmx file has no floppy mounted, but the additional vmx_data_post config entries.