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: Ica879c31574060471fb22c53761e6d4549a05db2

setup.cfg

@@ -1,11 +1,11 @@
 [metadata]
 name = magnum_tempest_plugin
 summary = Tempest plugin for Magnum Project
-description-file =
+description_file =
     README.rst
 author = OpenStack
-author-email = openstack-discuss@lists.openstack.org
-home-page = https://opendev.org/openstack/magnum-tempest-plugin
+author_email = openstack-discuss@lists.openstack.org
+home_page = https://opendev.org/openstack/magnum-tempest-plugin
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology