Update YAML schema to include `workflow` specification

In order to generalize workflow [checkout |install | run] tools, we can edit the yaml to make the workflow repo and tag user specified. This allows flexibility to extend these tools to work for multiple workflows (not just pp)
Example:
```
workflow:
    run:
        repo: (str)
        tag: (str)
    pp:
        repo: (str)
        tag: (str)
```