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.

Mandatory ```vm_folder``` doesn't allow me to use this post-processor.
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 create new VM folders. 
So I had to make this change in Packer code, to run OVFTool with an empty value.

```datastore```, and ```vm_network``` are not showstoppers, but they can be also skipped, just to make templates simplier.