Update block_retired.py to work against staging.
    
    A --staging option is added that does 2 things:
    
    - Points the script at staging services like koji.stg.fp.o and
      admin.stg.fp.o/pkgdb.
    - Limits the branches on which packages get retired in staging by
      default.  This is necessary only because, for instance, production
      koji seems to have el5 defined while staging koji does not.