Astarte Operator shouldn't handle RabbitMQ resources

[matt-mazzucato]

The RabbitMQ cluster operator takes care of handling a RabbitMQ instance along with all its configurations and, as such, the Astarte Operator should stop handling RabbitMQ related resources. As long as new versions of RabbitMQ are available, there might be the need of changing the Operator's logic just mirroring some of the functionalities implemented within the RabbitMQ Operator, leading to a useless overhead.

This also implies a rework of the current Astarte CRDs.