[api] Do not use class variables but global variables

bunny will start a thread - and it might be good to actually start this
thread on initial startup, but this would be specific to the web server
used

src/api/app/models/event/send_to_bus.rb

@@ -1,9 +1,11 @@
 module Event
   # performed from delayed job triggered by clockwork
   class SendToBus
-    def self.connection(config)
-      @@conn ||= Bunny.new(config['url'], log_level: Logger::DEBUG)
-      @@conn.start
+    def connection(config)
+      return $rabbitmq_connection if $rabbitmq_connection
+      $rabbitmq_connection = Bunny.new(config['url'], log_level: Logger::DEBUG)
+      $rabbitmq_connection.start
+      return $rabbitmq_connection
     end
 
     def amqp_config
@@ -14,7 +16,7 @@ def bus_topic(config)
       # no config, nil topic
       return if config.empty? || config['url'].empty?
 
-      ch = self.class.connection(config).create_channel
+      ch = connection(config).create_channel
       # this has to be a predefined topic
       ch.topic(config.fetch('topic', 'pubsub'), persistent: true, passive: true)
     end