Use Snapshot Repos By Default

In the past, Cisco OpenStack Installer releases have generally pointed users at the stable pocket of our apt repositories so that users have easy access to new updates as they are released (similar to the model used by, for example, Ubuntu Linux). Release notes always include instructions on how to switch to using a snapshot repository if users would prefer to stay locked on a fixed version of code. However OpenStack has proven to be such a fast moving target that we're hearing increasing requests to inverse that model: ship the release configured to use the snapshot repo and provide instructions on how to switch to the stable pocket. This prevents any problems with changes in the composition layer catching users who don't read the release notes unawares.