Handle parent class of ClusterComputeResource.

I'm not super familiar with the vsphere api or rbvmomi but I'm getting some complaints about unknown types in listing pools. Seems the api is returning ComputeResources instead of the expected ClusterComputeResource. As ClusterComputeResource extends ComputeResouce I let it match it.
```
Switched to branch 'master'
mkent@mbp master ~/git/knife-vsphere> knife vsphere pool list
Folder:
ComputeResource: 10.10.0.36
Unknown type ComputeResource, not enumerating
ComputeResource: 10.10.0.35
Unknown type ComputeResource, not enumerating
ComputeResource: 10.10.0.37
Unknown type ComputeResource, not enumerating
mkent@mbp master ~/git/knife-vsphere> git checkout fix-compute-ref
Switched to branch 'fix-compute-ref'
mkent@mbp fix-compute-ref ~/git/knife-vsphere> knife vsphere pool list
Folder:
ComputeResource: 10.10.0.36
ComputeResource: 10.10.0.35
ComputeResource: 10.10.0.37
```