MQTT Agent - add publish with RETAIN flag as option

[kg333]

The MQTT Agent currently appears to have no support for publishing messages with MQTT's RETAIN flag feature, which directs the MQTT broker to keep a copy of the last message for new subscribers to a topic.

ruby-mqtt does support RETAIN by sending an additional argument to the MQTT::Client#publish function, and editing mqtt_agent.rb to force RETAIN does appear to work for my use case:

huginn/app/models/agents/mqtt_agent.rb

Line 112 in 4d17829

c.publish(interpolated(event)['topic'], event.payload['message'])

 c.publish(interpolated(event)['topic'], event.payload['message'], retain=true) 

I suspect not everyone would be content with every MQTT agent publishing only with RETAIN, so this issue is to suggest adding a configuration option on an agent-by-agent basis. Based on some of the discussion in #1289, this may not be as simple since there's complications due to MQTT QoS level.