Add volume total to Mgmt Instance Get

Add volume.total attribute and value to instance info returned by
mgmt show/get call in addition to volume.used.

This will help in validating cinder volume size reflects actual
filesystem size.

Change-Id: Ia89269edbf90b91967dedcc787dd54f8a56695de
Implements: blueprint add-volume-total-attribute
Merge "Add volume total to Mgmt Instance Get"