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: I01e07a0f954a61cc4fa645bbd19a239a71070ba8

setup.cfg

@@ -1,13 +1,13 @@
 [metadata]
 name = tripleo-image-elements
 summary = Disk image builder elements for deploying OpenStack.
-description-file =
+description_file =
     README.rst
 author = OpenStack
-author-email = openstack-discuss@lists.openstack.org
+author_email = openstack-discuss@lists.openstack.org
 license = Apache License (2.0)
-home-page = https://opendev.org/openstack/tripleo-image-elements
-python-requires = >=3.6
+home_page = https://opendev.org/openstack/tripleo-image-elements
+python_requires = >=3.6
 classifier =
     Environment :: OpenStack
     Development Status :: 3 - Alpha