u/fjammes/dm-1446

Reviewing DM-1446
Looking at the `STEP_DOC` definition is not obvious which steps they comment, `STEP_LIST` is constructed from several components and it needs attention to find matching step. I'd make this more explicit:

    STEP_DOC = {
        PREPARE: "create qserv_run_dir and attach it to current Qserv instance",
        DIRTREE: "create directory tree in qserv_run_dir", 
        ...

and so on.