introduce the CHRIS_ENV_PYTHONPATH

it is used to add some extra python libraries to the path, that might not be available on the cluster.

NOTE - DOESNT WORK ON A NON SHARED FILESYSTEM CONTEXT
Use CHRIS_PLUGINS_FOLDER_NET to run viewer plugin

CHRIS_PLUGIN_FOLDER might not be available from the cluster.

Also add CHRIS_ENV_PYTHONPATH to the PYTHONPATH in chris.env