Add feed for OpenStack-related hastexo announcements

This feed points to posts on hastexo web site that are tagged both
"openstack" and "announcement", and thus exposes only official
announcements related to OpenStack. Blog posts, tagged only with
"openstack", will continue to be included in individual bloggers' feeds.

Change-Id: I3046b39eb219d3c21bac93c78965ee50e13e6154