setup.cfg: Replace dashes with underscores
setup.cfg: Replace dashes with underscores

Resolves warnings like the following:

  UserWarning: Usage of dash-separated 'description-file' will not be
  supported in future versions. Please use the underscore name
  'description_file' instead

Change-Id: Iafa5424d5c337226f5795f6e79b530f4e8b499e2

openstack/ceilometermiddleware