JVB instance should be recognized by MUC_NICKNAME instead of USERNAME

[slykar]

To make the configuration of JVB auto-scaling easier, you can reuse the same username and password for JVB to enter the MUC room. This is even suggested by the maintainers on the Jitsi forums. The thing that needs to be unique is the MUC_NICKNAME.

Right now the exporter identifies instances by the USERNAME, which in my case is always jvb. It would be great if the instance was recognized by the MUC_NICKNAME instead, which in my case is the machine hostname assigned by AWS, e.g. ip-10-0-0-78.

Note: I'm running jitsi-videobridge2