AMQP-424 PlaceHolder Resolution in @RabbitListener

JIRA: https://jira.spring.io/browse/AMQP-424

Place holder resolution in @RabbitListener arguments

This commit allows to use place holder definitions for RabbitListener
attributes, effectively allowing to externalize those settings from
the code.

Issue: AMQP-424

Improve priority configuration

This commit allows to configure the priority attribute with a place
holder. It also defaults to not change the priority at all if it is
not set. Update the Javadoc to indicate that the value can be negative
as well.

Polish tests.