Force to use markdown to prevent pypi issue

Recently the pypi team have reintroduce some deactivated tests
on package deployment and especially tests concerning long_description
syntaxe.

If projects don't specify that they use the markdown format the
package deployment will fail if something went wrong in the markdown format.

By example if a project use inline literal this can be an issue due to
the fact that if `long_description_content_type` is missing [1]. The
default format will be `text/x-rst` where code block is different from
markdown.

These changes force to use the markdown format to avoid this kind of issue
on project who use `README` at markdown format.

[1] http://lists.openstack.org/pipermail/openstack-discuss/2019-July/007459.html

For more details:
pypi/warehouse#5890 (comment)

Change-Id: I6eb24615b08d1d2f8a90bb1d871256fe9abcd99f

setup.cfg

@@ -2,6 +2,7 @@
 name = puppet-pacemaker
 summary = Puppet module for Pacemaker
 description-file =
+long_description_content_type = text/markdown
     README.md
 author = Mirantis, Red-Hat and OpenStack Contributors
 author-email = openstack-discuss@lists.openstack.org