Fix DB path in config
    
    Apparently, for it to really be interpreted as an absolute path, you
    need:
    
      sqlite:////my/absolute/path/db.sqlite
    
    This:
    
      sqlite:///my/absolute/path/db.sqlite
    
    gets interpreted as "db.sqlite" in the current dir.
    
    This isn't a problem in practice because glance-registry gets run in
    /var/lib/glance, but if you run it from the command line in a different
    dir you see an error.