fix 'type' object cannot be interpreted as an integer

[spaceone]

PDB stacktrace:

 plugins/twitter_bootstrap_rst_directives/bootstrap_rst_directives.py(369)run()                                                                                                                  
-> child.set_class('alert-link')
> tmp/lib/python3.7/site-packages/docutils/nodes.py(1072)set_class()
-> DeprecationWarning, stacklevel=2)

The deprecated function code:

  1067     def set_class(self, name):
  1068         """Add a new class to the "classes" attribute."""
  1069         warnings.warn('docutils.nodes.Element.set_class() is deprecated; '
  1070                       ' and will be removed in Docutils 0.21 or later.',
  1071                       "Append to Element['classes'] list attribute directly",
  1072                       DeprecationWarning, stacklevel=2)
  1073         assert ' ' not in name
  1074         self['classes'].append(name.lower())