Moved model-based dataset generator tools to common

 * Adds datasets.py in cloudcafe/common
 * Adds methods for helping create dataset generators
   from model lists.
 * Adds compute dataset generators for images,
   flavors, and images_by_flavor

Change-Id: I6480f0c61b04183514e49081fe96abbfbc57f82c
Merge "Moved model-based dataset generator tools to common"