Change URL of the Barbican documentation

Instead of the wiki on Github the developer documentation
of barbican (http://docs.openstack.org/developer/barbican/)
should be used.

Change the reference link from https://github.com/cloudkeep/barbican
to http://docs.openstack.org/developer/barbican/.

Change-Id: I27cac61eb4973bdf6c8f1ec3cd0e8e3fe6f11bb7
Merge "Change URL of the Barbican documentation"