cumulus-util: Automatically set Python search path.

Attempt to set the Python library search path so that cumulus-util can find
the cumulus Python modules, without PYTHON_PATH having to be set
explicitly.

Modules are looked for in the "python" directory where the cumulus-util
binary resides; this is appropriate for running cumulus-util directly from
the source code directory, but may not be if the tools are installed
somewhere else.