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: I136843f66db8d10f59fa95bd69c87c7dde15632c

setup.cfg

@@ -1,7 +1,7 @@
 [metadata]
 name = mistral-dashboard
 summary = Mistral dashboard
-description-file = README.rst
+description_file = README.rst
 license = Apache License, Version 2.0
 classifiers =
     Programming Language :: Python
@@ -15,11 +15,11 @@ classifiers =
     License :: OSI Approved :: Apache Software License
     Operating System :: POSIX :: Linux
 author = OpenStack
-author-email = openstack-discuss@lists.openstack.org
-home-page = https://docs.openstack.org/mistral/latest/
+author_email = openstack-discuss@lists.openstack.org
+home_page = https://docs.openstack.org/mistral/latest/
 
 [global]
-setup-hooks =
+setup_hooks =
     pbr.hooks.setup_hook
 
 [files]
@@ -27,4 +27,4 @@ packages =
     mistraldashboard
 
 [upload_sphinx]
-upload-dir = doc/build/html
+upload_dir = doc/build/html