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

mbrukman/cloud-launcher,mbrukman/cloud-launcher,mbrukman/cloud-launcher,mbrukman/cloud-launcher