Always provide a node name to rabbitmq

When booting, the processes expect that whatever host is in the
"node name" is resolvable by DNS. Disabling reverse DNS lookups does
not refute this requirement. This change provides an environment
variable that RabbitMQ is expecting to find with a value that should
be resolvable on all machines: `localhost`.

src/modules/services/rabbitmq.nix

@@ -141,6 +141,7 @@ in
     env.RABBITMQ_CONFIG_FILE = config_file;
     env.RABBITMQ_PLUGINS_DIR = concatStringsSep ":" cfg.pluginDirs;
     env.RABBITMQ_ENABLED_PLUGINS_FILE = plugin_file;
+    env.RABBITMQ_NODENAME = "rabbit@localhost";
 
     processes.rabbitmq.exec = "${cfg.package}/bin/rabbitmq-server";
   };