Automatic merge from submit-queue

Add admission controller for default storage class.

The admission controller adds a default class to PVCs that do not require any
specific class. This way, users (=PVC authors) do not need to care about
storage classes, administrator can configure a default one and all these PVCs
that do not care about class will get the default one.

The marker of default class is annotation "volume.beta.kubernetes.io/storage-class", which must be set to "true" to work. All other values (or missing annotation) makes the class non-default.

Based on @thockin's code, added tests and made it not to reject a PVC when no class is marked as default.
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@kubernetes/sig-storage