Proposal for deployment of default StorageClass

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+# Deploying a default StorageClass during installation
+
+## Goal
+
+Usual Kubernetes installation tools should deploy a default StorageClass
+where it makes sense.
+
+"*Usual installation tools*" are:
+
+* cluster/kube-up.sh
+* kops
+* kubeadm
+
+Other "installation tools" can (and should) deploy default StorageClass
+following easy steps described in this document, however we won't touch them
+during implementation of this proposal.
+
+"*Where it makes sense*" are:
+
+* AWS
+* Azure
+* GCE
+* Photon
+* OpenStack
+* vSphere
+
+Explicitly, there is no default storage class on bare metal.
+
+## Motivation
+
+In Kubernetes 1.5, we had "alpha" dynamic provisioning on aforementioned cloud
+platforms. In 1.6 we want to deprecate this alpha provisioning. In order to keep
+the same user experience, we need a default StorageClass instance that would
+provision volumes for PVCs that do not request any special class. As
+consequence, this default StorageClass would provision volumes for PVCs with
+"alpha" provisioning annotation - this annotation would be ignored in 1.6 and
+default storage class would be assumed.
+
+## Design
+
+1. Kubernetes will ship yaml files for default StorageClasses for each platform
+   as `cluster/addons/storage-class/<platform>/default.yaml` and all these
+   default classes will distributed together with all other addons in
+   `kubernetes.tar.gz`.
+
+2. An installation tool will discover on which platform it runs and installs
+   appropriate yaml file into usual directory for addon manager (typically
+   `/etc/kubernetes/addons/storage-class/default.yaml`).
+
+3. Addon manager will deploy the storage class into installed cluster in usual
+   way. We need to update addon manager not to overwrite any existing object
+   in case cluster admin has manually disabled this default storage class!
+
+## Implementation
+
+* AWS, GCE and OpenStack has a default StorageClass in
+  `cluster/addons/storage-class/<platform>/` - already done in 1.5
+
+* We need a default StorageClass for vSphere, Azure and Photon in `cluster/addons/storage-class/<platform>`
+
+* cluster/kube-up.sh scripts need to be updated to install the storage class on appropriate platforms
+  * Already done on GCE, AWS and OpenStack.
+
+* kops needs to be updated to install the storage class on appropriate platforms
+  * already done for kops on AWS and kops does not support other platforms yet.
+
+* kubeadm needs to be updated to install the storage class on appropriate platforms (if it is cloud-provider aware)
+
+* addon manager fix: https://github.com/kubernetes/kubernetes/issues/39561