Add a default cache version.
None doesn't work in a complex value.

Setting a default value and adding a changelog entry.
Merge pull request #1634 from edx/feanil/version_default_cache

Add a default cache version.
Switch from legacy variable templating system to Jinja2 everywhere
Add spacing to match coding conventions.
Add back dollarsigns to the vars that need it.
Remove last known dollar vars.
Upgrade ansible and its dependencies.