Delete workspaces when deleting a job

When working with many short-lived feature branches, Jenkins can start to eat up a lot of disk space by leaving old workspaces around from old builds.

This seems to be especially true with the Pipeline Multibranch Plugin. When that plugin deletes the job for a branch when the branch is deleted, it should simultaneously clean up any of the workspaces created for that branch job (e.g. "feature-foo", "feature-foo@2", "feature-foo@3", etc).