Add envs for different sqlalchemy versions

Adjust tests to skip the sqlalchemy test if
sqlalchemy is not installed. Adjust examples
to fallback to a directory based backend if
the sqlalchemy does not load or is not available.

Include a updated tox.ini (generated from the
toxgen.py script) that includes the new venv
variations.

Change-Id: I7686f09901a9b65d7c81b4e037b5bffc24aa7ef7
Merge "Add envs for different sqlalchemy versions"