setup.cfg: Replace dashes by underscores

Since setuptools v54.1.0[1], the parmeters with dash have been
deprecated in favor of the new parameters with underscore.

This change updates the parameters accordingly to avoid the warnings
like the example below.

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

[1] https://github.com/pypa/setuptools/commit/a2e9ae4cb

Change-Id: Ie59fc55bdb2f8ac20ac4a7a6718e9cdb27c4ad8a

setup.cfg

@@ -1,11 +1,11 @@
 [metadata]
 name = magnum-ui
 summary = Magnum User Interface
-description-file =
+description_file =
     README.rst
 author = OpenStack
-author-email = openstack-discuss@lists.openstack.org
-home-page = https://docs.openstack.org/developer/magnum-ui/
+author_email = openstack-discuss@lists.openstack.org
+home_page = https://docs.openstack.org/developer/magnum-ui/
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology