As of setuptools 36.4.0, support for Description-Content-Type
metadata was added via the long_description_content_type variable.
Add a metadata.description-content-type option for setup.cfg so it
can be provided through PBR. This makes it possible to instruct PyPI
and potentially other frontends to know an explicit RFC 1341 content
type, along with character set and variant, for the accompanying
long description.
Change-Id: I852f603d9ca4e4cd469eab4cae9933ef2581e3af