OSReactions: Use a category for OS planet

In order to manage posts publication frequency, we created a new
category under the which we'll publicizing posts meant to be read by
openstack's planet.

This category doesn't have any post yet, which makes this patch safe
since old posts won't be read again by the planet.

Change-Id: I662d3f3671dec72b817db99c733e3532975f12c1