Clear the OS scripts environment

The OS scripts currently run with the whole noded environment; this is
different from the hooks which run with a cleared one and most likely
an oversight.

This _might_ create problems when upgrading, so it needs to be clearly
announced for the new version.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>