Update base scenarios for the Mitaka release.

I've updated the two basic scenarios to support Mitaka release.

This PR is about starting the discussion about :

• The mitaka support
• The way we reorganize the git to support the future releases of OpenStack

For the second item I propose to

• create a branch to support the liberty release
• keep master up-to-date with the latest release (here Mitaka)

In the future old release will be supported in dedicated branches and current release will be in master.