Housekeeping vhost management envs are misnamed

Since https://github.com/astarte-platform/astarte/pull/1475, Housekeeping is able to create a new RabbitMQ vhost for each Astarte realm. This is possible via the RMQ HTTP API. However, envs that allow to set the RabbitMQ URL are named like HOUSEKEEPING_AMQP_*, which is misleading because it seems to relate to AMQP and not HTTP. Rename the envs to more clearly match their purpose, e.g. HOUSEKEEPING_RABBITMQ_MANAGEMENT_*.

Note that this also implied we're tied to RabbitMQ as AMQP broker, as long as we use RMQ HTTP APIs for vhost creation.