Avoid KeyError: 'CONFIG_CINDER_VOLUMES_SIZE' error

- When CONFIG_CINDER_VOLUMES_CREATE=n, it should always pass in the cinder
  volume size calculation block.

Change-Id: I6cf6fd4767c66dc87ea31cbafe17bb8fcfebbf70
Fixes: rhbz#1164359
Signed-off-by: Gael Chamoulaud <gchamoul@redhat.com>