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>.

[prometheus addon] Fix missing storage class in alertmanager PVC

Fixes ci-kubernetes-e2e-gce-prometheus test suite
https://k8s-gubernator.appspot.com/builds/kubernetes-jenkins/logs/ci-kubernetes-e2e-gce-prometheus
This PR adds missing storage class to PVC, gce does not have default storage class, so k8s was not able to create PVC.
```release-note
NONE
```