telgraf: Fix config folder permissions
With the recent update to telegraf's container image it now runs under its own user and thus we need to change the permissions to make the config file accessible for the new user.
See https://github.com/influxdata/influxdata-docker/pull/536
IMPORTANT: I also changed the image variant to alpine, because that matches my config and it matches the values i put in as default for the permissions. You may need to look up the uid/gid for the normal image and change them. If you know which GID/UID the telegraf user has in the debian based image, I can also update my commit to match that if you need :)