Automatic merge from submit-queue. If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>..

Do not install metrics/v1alpha1 by default

We want to have `metrics/v1alpha1` in the repo in order to support the previous version of HPA, but we don't want to install them by default.

ref https://github.com/kubernetes-incubator/metrics-server/pull/15