proxy mosquitto websockets in nginx to enable using TLS on mqtt

[maaikelimper]

We currently support TLS-encryption using local SSL-certificates on the wis2box-instance but this only applies to HTTP.

I propose we update nginx to add a proxy to the mosquitto websockets port

This will also resolve issues I encountered due to firewalls commonly blocking ports other than 80/443

We should also add some documentation for how users using SSL-certificate via a proxy-server can setup proxy to the wis2box-broker websockets port