Add a graphite key for all jobs in a pipeline

Add a key for convenience that rolls up all of the job completions in a
pipeline, so that such a graph can be built with only about 10 keys.

Comes from OpenStack I93ef1dca350cd66d880e8cb7f4478c47dbc5960e

Change-Id: Ibda87cf9b54efb18ace5860813421ccf66e09bec