Link to os-cloud-config developer docs

Now that os-cloud-config's developer docs are in a state that I'm
happy with them being rather more discoverable, link to them in
openstack-projects.html, in a new section titled "OpenStack related
projects".

Change-Id: Ifb8b92e9b51ac968febde4b24334334323bf0638
Merge "Link to os-cloud-config developer docs"