allow easy clustering of the whole workflow without associating labels for the jobs

label based clustering works if jobs are associated with labels.
for the case, where user wants to cluster the whole workflow and run it using PMC, we recommend users to set the same label value to the labels for all the jobs in the workflow. However, not all workflow generators associate labels with the jobs in the workflow.

There should be an easier way for users to support this, as clustering the whole workflow means associating the same label with all the jobs.