[RabbitMQ] Allow adding custom startup scripts

[DiamondJoseph]

For my RabbitMQ instance, I need a second user without admin permissions (previously I had an admin account with permission for the management console, with password mounted from a secret and a user account for sending/reading from all queues with "guest").

This was being managed by running all scripts mounted in a named ConfigMap at startup, and defining that ConfigMap from a short shell script. I've only briefly looked at the definition of the initContainer and the scripts ConfigMap, so I may have missed something obvious, but I would like a way to achieve some of the same behaviour without losing what the current init scripts are doing.