Add tini to the telegraf docker image

[tim-basic-fit]

An init process will guard against zombie process issues, using tini is the standard approach for containers. This fixes #479.

If the exec plugin is used, it's possible to get a ton of zombie processes if the exec scripts get stuck for whatever reason.

Design decisions:

• The -g option: Ensures the entire zombie process tree gets killed, far more robust that way
• Why not use the built-in tini (docker run --init): Not all container runtimes support this, this ensures there's a safe default. --init is not supported in most k8s setups.