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: Ib3752e0165c296d7e052ebe02381c448a233c628
diff --git a/setup.cfg b/setup.cfg
index 8776610..a25d96c 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -1,11 +1,11 @@
 [metadata]
 name = heat-tempest-plugin
 summary = OpenStack Orchestration Tempest Plugin
-description-file =
+description_file =
     README.rst
 author = OpenStack
-author-email = openstack-discuss@lists.openstack.org
-home-page = http://docs.openstack.org/developer/heat-tempest-plugin
+author_email = openstack-discuss@lists.openstack.org
+home_page = http://docs.openstack.org/developer/heat-tempest-plugin
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology