add user identity provisioning

@liggitt This builds on top of https://github.com/openshift/origin/pull/334, so I'll rebase it again once yours goes in, but I'm still looking for feedback.
we should be consistent in logging even thought this is V(4), otherwise the logging output will be a mess in the end. I would pickup a single sentence, like `glog.V(4).Infof("Got userIdentityMapping: %#v", identity)` and stick with it.