Remove pinning

Issue DE102

Since we want to start using features in rabbitmq-server 3.0.4, let's default to using the non-distro version in both CentOS and Ubuntu.  This change removes the pinning configuration for apt so that a manual apt-get upgrade doesn't attempt to DOWNGRADE rabbitmq-server.

If someone wants to run their distro-provided rabbitmq-server package, you can set ['rabbitmq']['use_distro_version'] to true.  See the upstream rabbitmq cookbook for more info.