WIP: Persistent disks proposal

This is a draft proposal for expanding GCEPersistentDisk to a fuller PersistentDisk framework.
@smarterclayton correct, there is no "local+shared" disk, but there could be two types of storage (based on perf) from the provider.  Just as EBS has 3 types, I was thinking our "enterprise provider" (for lack of a better term) could offer two types: local and attached (node local and nfs).   But that also means anyone using that provider is now required to use an NFS server, which might not be correct for their environment.

Cloud providers are easy to write for, but a heterogeneous enterprise customer is more difficult.  I believe the correct solution is to make it easy to customize the storage provider per environment.  A little bit of Spring dependency-injection type of configuration would be lovely.  Let the admin configure the environment and allow devs to request normalized types of disks.