Feanil/optional datadog

@jzoldak @benpatterson 

I moved around the datadog dependency because I don't need it for the admin jenkins we use in devops and so I wanted it to be optional.  I also updated the jenkins_master.yml play you're using to indicate that it is being used by testeng.
As a part of this change, I'm now using it within the `jenkins_master/defaults/main.yml`