Define and use PV and PVC metadata keys used by external provisioner.
The CSI external-provisioner (v1.6.0+) introduces the
--extra-create-metadata flag, which automatically sets map<string, string>
parameters in the CSI CreateVolumeRequest.
Add utility functions to set/Get PV/PVC/PVCNamespace metadata on image
Signed-off-by: Prasanna Kumar Kalever <prasanna.kalever@redhat.com>