Fix: post-processor for vSphere not working

I fixed the post-procoessor for vSphere as it was not able to upload the vm.

Important: I changed the usage and separated the cluster and resource_pool variable, see sample config for details.

```
  "post-processors": [
    {
      "type": "vsphere",
      "vm_name": "NewVM",
      "vm_folder": "AnyFolder",
      "host": "host.domain.tld",
      "username": "user",
      "password": "secret",
      "datacenter": "DC",
      "datastore": "ds1",
      "vm_network": "dev_network",
      "cluster": "VMCluster",
      "resource_pool": "Infrastructure Pool",
      "insecure" : true,
      "debug" : true,
      "only": ["vmware"]
    }
  ]
```