Retained flag for helpers not settable since 0.8.4

[jneilliii]

What were you doing?

Using MQTT plugin's helper to publish with the retained flagged as true and documented on the readme.

What did you expect to happen?

The retained message to be set appropriately per helper call.

What happened instead?

Error as described below to changes to the helpers.

mqtt_publish() got an unexpected keyword argument 'retained'

Version of OctoPrint

Any supported version

Version of the MQTT plugin

0.8.6

Used MQTT broker and its version

N/A

Link to octoprint.log

Unnecessary

Link to contents of Javascript console in the browser

N/A

Screenshot(s)/video(s) showing the problem

N/A

Possible Solution

Revert making retain flags globally in helper functions and put them back into the publish calls themselves, which would also make them a granular option per event, which would also resolve #76