ncm-gsissh: make compatible with recent version of gsissh

- Execute the setup script only if it exists
- Do not hack the startup script if it doesn't exist

Fixes https://github.com/quattor/template-library-grid/issues/83

There is no attempt to improve this component that should probably be deprecated in a future release. Just an attempt to make it a not too bad citizen (not returning errors for things that don't have sense anymore) with a very minimalistic modification.