Make Jsonnet optional; only require it if we're parsing a Jsonnet file.

Previously, loading this module would fail without the presence of _jsonnet.so
in PYTHONPATH leading the user unable to do anything without Jsonnet, even when
not using Jsonnet. With this change, lacking Jsonnet still allows user to parse
and evaluate Python and YAML files.
Added auto-generation of the VM images offline cache.
Use auto-generated VM images cache data instead of hand-written data.
Show overall status after running all tests; exit with proper code.
Exit with non-zero status code on error to propagate failures.