Unpin disk-image-builder and bring back cloud-init

* The reddwarf-guest element was purging the cloud-init package
  which is required to correctly size the root partition
* unpins disk-image-builder so we can build from tip once again

fixes bug 1175878

Change-Id: I3b2f702302051827d84e20fa1b8e76e4ad1d6b92
Merge "Unpin disk-image-builder and bring back cloud-init"