Add empty response for ValidateVolumeCapabilities
Add empty response for ValidateVolumeCapabilities

The ValidateVolumeCapabilities is required to be implemented for the
Controller service. This is the last missing required API. Right now it
just returns an empty response, which is allowed. Since the "confirmed"
value is empty, this tells the CO that it has *not* confirmed that any
of the requested capabilties for the given volume have been confirmed.

kubernetes/cloud-provider-vsphere,kubernetes/cloud-provider-vsphere