Feature 10972: Support of volume multi-attach

This change describes the new keys and values than can be used in a VDU
storage volume to identify that the volume can be attached to several
virtual machines.

Change-Id: If271910f7c0ecebc07cf7e8405b15861b45c1e19
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>