code: def db_user_exists(username):
      
      qry = u"""SELECT COUNT(*) FROM pg_roles where rolname = \'{username}\';"""
      output = StringIO()
      excute_query(
          qry.format(username=username),
          flags="-Aqt",
          use_sudo=True,
          stdout=output
      )
      # FIXME: is there a way to get fabric to not clutter the output
      # with "[127.0.0.1] out:" on each line?
      lines = output.getvalue().splitlines()
      return lines and lines[0].endswith('out: 1')
docs: Return True if the DB user already exists.