Deploy a default StorageClass on all clouds

This is part of proposal kubernetes/community#285.

kops should deploy a default storage class on all platforms, not only AWS. Kubernetes will ship storage classes in kubernetes.tar.gz and kops should just look if there is one in `cluster/addons/storage-class/XXX/default.yaml` there and install it, e.g. via addon manager.

Start by reading the proposal kubernetes/community#285 and the storage-class addon path, cluster/addons/storage-class/XXX/default.yaml. Trace how kops installs addons and how platform-specific storage classes are selected. Done means a default StorageClass is deployed on every supported cloud platform.

Written by the indexing model from the issue text.