do not ignore attributes

The jmx.yaml.erb file has a small typo which causes it to ignore attributes. This fixes that issue. I didn't notice any tests for the jmx.yaml file...

I also realized that the structure of the yaml file doesn't appear to fully support both invocations of the attributes referenced [here](http://docs.datadoghq.com/integrations/java/).

I was thinking it would be helpful if the attributes key could be an array or a hash. The array would be a list of strings, naming attributes. The hash would be a hash of hashes, the key would define the attribute, and the hash would define the metric type and alias. If that's agreeable, I could add that to this PR or make a separate one (and update the Vagrantfile).