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

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

Change-Id: I59551c46d943fdc5224721a8f0d8003c0ac7e8d9

setup.cfg

@@ -1,11 +1,11 @@
 [metadata]
 name = mistral_tempest_tests
 summary = Tempest plugin for Mistral project
-description-file =
+description_file =
     README.rst
 author = OpenStack
-author-email = openstack-discuss@lists.openstack.org
-home-page = https://opendev.org/openstack/mistral-tempest-plugin
+author_email = openstack-discuss@lists.openstack.org
+home_page = https://opendev.org/openstack/mistral-tempest-plugin
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -29,13 +29,13 @@ tempest.test_plugins =
     mistral_test = mistral_tempest_tests.plugin:MistralTempestPlugin
 
 [build_sphinx]
-all-files = 1
-warning-is-error = 1
-source-dir = doc/source
-build-dir = doc/build
+all_files = 1
+warning_is_error = 1
+source_dir = doc/source
+build_dir = doc/build
 
 [upload_sphinx]
-upload-dir = doc/build/html
+upload_dir = doc/build/html
 
 [compile_catalog]
 directory = mistral_tempest_tests/locale