add "admission" API group

This commit is an initial pass at providing an admission API group.
The API group is required by the webhook admission controller being
developed as part of https://github.com/kubernetes/community/pull/132
and could be used more as that proposal comes to fruition.

hack/lib/init.sh

@@ -79,6 +79,7 @@ KUBE_NONSERVER_GROUP_VERSIONS="
  abac.authorization.kubernetes.io/v1beta1 \
  componentconfig/v1alpha1 \
  imagepolicy.k8s.io/v1alpha1\
+ admission.k8s.io/v1alpha1\
 "
 
 # This emulates "readlink -f" which is not available on MacOS X.