Update keystone readme to point to specs.o.org

There is at least one dead link in the README, and a few links to
the old identity-api on github.com. These should be linked to
specs.o.org/keystone-specs

Change-Id: Ida6eb0a48ff2f2518d3352624cc461c9ff2c458f
Merge "Update keystone readme to point to specs.o.org"