setup metadata: fix description
setup metadata: fix description

Our landing page on PyPI was not displaying the longer version of the
README, just a one-sentence summary. It turns out that `pbr` can indeed
read the README file, but specifying the `description` overrides that
with no warning. Yay.

Change-Id: Ic03412928fe09f5edab4a7b9f4297a485a740cd0

Telecominfraproject/gnpy