Adding abiltity to alter MQTT version using either envVars or ConfigFile

[JonahLuckett]

May be redundant due to the current inability to use MQTTv5 but this way we're ready for the node update if we wanted to be.

[durera]

After various discussions today, have decided that we'll support MQTTv5 by making it a replacement for v3.1.1 rather than an alternative/configuration option ... supporting multiple versions will create additional overheads in terms of testing and code paths that we'll need to maintain as we start to take advantage of the v5 features, so after consideration we're going to tweak how we do this. Will discuss with you @JonahIBM once you're back from xmas hols .. should be a few simple tweaks we can make here to make it so that "it's just MQTT v5"