Optional parameters in vSphere post-processor

```datastore```, ```vm_folder```, and ```vm_network``` become optional and can be omitted in templates.
Technically, they are not required by OVFTool.

In my environment VM folders are not used, and there can be no valid value for this option. I'm not an administrator of our vSphere server, and don't have permissions to chreate new VM folders. So I had to make this change in Packer code, to run OVFTool with empty value for ```vm_folder```.