Use git.openstack.org everywhere
Use git.openstack.org everywhere

Our official git master is at git.openstack.org, update places
that use github instead.

Also, update content to point to main page, the user guide is not in
DocBook anymore.

Also, remove some mention of Rackspace, they do not use this anymore.

Change-Id: Ic63d95bcd409ccd58255e21a2098683d70c6b83f

stackforge/clouddocs-maven-plugin,stackforge/clouddocs-maven-plugin,stackforge/clouddocs-maven-plugin,stackforge/clouddocs-maven-plugin