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.
The only place this `COMMON_ENABLE_DATADOG` is used is iat the play level so I think you would want to add a `when: COMMON_ENABLE_DATADOG` condition to the role include