Add scripts/ dir and integrations_scripts_dir config.

[malcyon]

This adds some automated setup to the enterprise image when using scripts in notifications.

• Add the /opt/graylog/data/scripts directory
• Add the integrations_scripts_dir property to graylog.conf

Similar change to https://github.com/Graylog2/fpm-recipes/pull/107 for the os packages.

Fixes #158