Added VNFD_SCHEMA_VERSION property in metadata for backward compatibility
requirements. Since the Dublin SDC DM does not support backward
compatibility with Casablanca SDC DM, we need a place to idenitfy the DM
version it uses in the csar template.

Change-Id: I9586d1f442b6d166384acfa5eb3ca969e110b1b0
Issue-ID: INT-1037
Signed-off-by: Lianhao Lu <lianhao.lu@intel.com>