WIP: Persistent disks proposal

This is a draft proposal for expanding GCEPersistentDisk to a fuller PersistentDisk framework.
Let's make the proposal be two core attributes (minimum size, and max writers) and then sketch out the other attributes (IOPS, throughput, latency) and propose how they might be reflected. 

----- Original Message -----
> > +
> > +* NFS shares will not be created dynamically
> > +    * The system administrator will create, format, and export NFS shares
> > into a pool.
> > +    * Scripts will be provided to make this task easier for admins.
> > +    * Responsibility for keeping the pool full is left to the
> > administrator.
> > +* NodePersistentDisk can crash a host
> > +    * Without quotas in place, any pod can fill the filesystem via an
> > EmptyDir, HostDir, or NodePersistentDisk.
> > +    * A filesystem like XFS provides project quotas.
> > + * All disks are considered "new" in Kubernetes
> > + 	* Migrating legacy data is not in scope
> > + 	* Existing named disks are supported by administrative action (adding
> > disk objects in etcd)
> > +
> > +
> > +## Size and Performance -- Requesting a disk
> > +
> > +A storage request is made using the API where only intent is expressed
> > (size and performance).  Kubernetes handles creating, attaching/mounting,
> > deleting, and otherwise managing the disk.
> 
> Fair enough.  I see the perf traits for the various disks in GCE and AWS.
> They don't match each others (in terms of values), but they have consistent
> types (IOPS, throughput, etc), so I can make an internal mapping from your
> requested disk to what the provider offers.
> 
> ---
> Reply to this email directly or view it on GitHub:
> https://github.com/GoogleCloudPlatform/kubernetes/pull/2609/files#r21829308