telegraf: remove unused mosquitto dependency

Seems like remnant misconfiguration from the beginning.

Fixes docker-compose.yml to be valid even if you don't choose mosquitto
fron the menu.

Added documentation about adding the mosquitto 'depends_on' back if you
use the mqtt_consumer addition.

Resolves #550

.templates/telegraf/service.yml

@@ -13,5 +13,4 @@ telegraf:
     - /var/run/docker.sock:/var/run/docker.sock:ro
   depends_on:
     - influxdb
-    - mosquitto
 

docs/Containers/Telegraf.md

@@ -244,6 +244,9 @@ $ docker-compose restart telegraf
 ```
 
 The `grep` strips comment lines and the `sudo tee` is a safe way of appending the result to `telegraf.conf`. The `restart` causes Telegraf to notice the change.
+This result in the Telegraf service depending on the Mosquitto service, as such
+you may want to edit `docker-compose.yml` and add `- mosquitto` to the
+`depends_on:` of the telegraf service.
 
 ## Getting a clean slate { #cleanSlate }