Currently the schema validation for attachment create assumes that
instance UUID will always be present in the request but that is
not the case when glance calls cinder for attachment.
Also there isn't any schema validation for MV3.54 which accepts
attachment mode in the request hence failing requests passing mode.
This patch removes the instance_uuid from required parameters and
adds schema validation for MV3.54.
Change-Id: I5108fd51effa4d72581654ed450d191a13e0e964
560318c82e