Allow cluster_name in log_line_prefix

I was recently surprised to notice that log_line_prefix doesn't support a cluster_name placeholder. I suggest adding one. If I don't hear objections I'll send a patch.

Before anyone asks "but why?!":

* A constant (short) string in log_line_prefix is immensely useful when working with logs from multi-node systems. Whether that's physical streaming replication, logical replication, Citus, whatever, it doesn't matter. It's worth paying the small storage price for sanity when looking at logs. 

* Yes you can embed it directly into log_line_prefix. But then it gets copied by pg_basebackup or whatever you're using to clone standbys etc, so you can easily land up with multiple instances reporting the same name. This rather defeats the purpose.

--