modify chef_user to support fetch by username or ext auth id

ping @opscode/server-team 

Dependency on https://github.com/opscode/oc_chef_wm/pull/98
``Warning: undefined callback function fields_for_fetch/1 (behaviour 'chef_object')``

I'm not sure if this will cause the build to fail (I think we removed warnings as errors), but I'd still rather avoid introducing the warnings. 

Now I'm going to see what it would take to  introduce -optional_callback in a behavior...