Open file limit not set correctly

If node['rabbitmq']['open_file_limit'] is set to anything other than the default, the resulting ulimit line is normally written in /etc/rabbitmq/rabbitmq-env.conf.  That file is usually sourced, and doing so usually prints out an error along the lines of "operation not permitted" when using rabbitmqctl, etc.

The correct place for the ulimit is in /etc/default/rabbitmq-server.  I tested this manually by applying the updated recipe to the default-centos-65 and default-ubuntu-1204 images.

I followed the same format for the minitest tests, but could not figure out how to execute them.  I'd appreciate a quick rundown on how that's supposed to work, as I'm pretty new to Chef and test-kitchen and am just learning them now.
Looks like we should use `node['rabbitmq']['open_file_limit']` here as it has default value `nil`.  `node['rabbitmq']['max_file_descriptors']` has default value `1024` and is only used when `node['rabbitmq']['job_control']` is `upstart`. :broken_heart: