Not every cluster has a default storage class and even if they do, we
don't always want to use that default storage class for our persistent
volumes. Instead, allow the user to specific this via the cluster
configuration itself for the three cases we require (zookeeper,
scheduler, and registry).
Change-Id: I948d57ce59d9b16c1c70fc52af2e22bd6131e6e2
ac2488d242