mqtt: client_id vs. base topic

[hessijames79]

According to the documentation, the configuration options for the mqtt client_id and the mqtt base topic are the same.

However, choosing a base topic such as site1/general/viseron would result in a client_id containing a slash which MAY be allowed by the server according to the specification but it is no MUST.

I suggest to add a new configuration option such as base_topic that could default to the value of client_id for backwards compatibility.

Patrick