Update documentation for HDS HUS cinder driver

This change propose an update in HUS documentation.
  - remove unused information
  - adds more details about the configuration of HUS volumes
  - rename driver files: as we there are new HDS drivers in the road, we
    rename the driver to avoid confusions

Change-Id: I0030b115d921f228881d3ce529d590f25f79f20a
Implements: blueprint hds-hus-update