code: def modify_conf(cfgfile, service_name, outfn):
      
  
      if not cfgfile or not outfn:
          print('ERROR: There is no config file.')
          sys.exit(0)
  
      options = service_options[service_name]
      with open(cfgfile, 'r') as cf:
          lines = cf.readlines()
  
      for opt in options:
          op = opt.get('option')
          res = [line for line in lines if line.startswith(op)]
          if len(res) > 1:
              print('ERROR: There are more than one %s option.' % res)
              sys.exit(0)
          if res:
              (op, sep, val) = (res[0].strip('\n').replace(' ', '').
                                partition('='))
              new_val = None
              if opt.get('is_list'):
                  # Value for this option can contain list of values.
                  # Append the value if it does not exist.
                  if not any(opt.get('value') == value for value in
                             val.split(',')):
                      new_val = ','.join((val, opt.get('value')))
              else:
                  if val != opt.get('value'):
                      new_val = opt.get('value')
              if new_val:
                  opt_idx = lines.index(res[0])
                  # The setting is different, replace it with new one.
                  lines.pop(opt_idx)
                  lines.insert(opt_idx, '='.join((opt.get('option'),
                               new_val + '\n')))
          else:
              # Option does not exist. Add the option.
              try:
                  sec_idx = lines.index('[' + opt.get('section') + ']\n')
                  lines.insert(sec_idx + 1, '='.join(
                      (opt.get('option'), opt.get('value') + '\n')))
              except ValueError:
                  print('Invalid %s section name.' % opt.get('section'))
                  sys.exit(0)
  
      with open(outfn, 'w') as fwp:
          all_lines = ''
          for line in lines:
              all_lines += line
  
          fwp.write(all_lines)
docs: Modify config file neutron and keystone to include enabler options.