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

Setuptools v54.1.0 introduces a warning that the use of
dash-separated options in 'setup.cfg' will not be supported
in a future version [1].
Get ahead of the issue by replacing the dashes with underscores.
Without this, we see 'UserWarning' messages
like the following on new enough
versions of setuptools:

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

Change-Id: If04bce0a49fcffcd11dae18973cbcf570009c2bb

openstack/tap-as-a-service,openstack/tap-as-a-service