Fix image-build to set DIB_CLOUD_INIT_DATASOURCES

Diskimage-builder now needs you to explicity call out the cloud-init
datasources using DIB_CLOUD_INIT_DATASOURCES, and by adding in the
cloud-init-datasources element.

This is to cater to change Ibcc3b86d6ca567a23f89b7a1a36bc713e444ef68

Change-Id: Iaef92a7036ac13f626ec387f95b7803bd03874c7
Closes-bug: 1329068