AMQP-383 Polishing - Conditional Declaration

JIRA: https://jira.spring.io/browse/AMQP-383

The previous solution did not work for conditional declaration
queues because the internal `RabbitAdmin` was not in the list
of admins that should declare the element.

Add a setter for the `RabbitAdmin` and provide an attribute
in the namespace.

Add a test case for conditional declaration.

Add documentation, what's new.