Initial framework for external volumes. 

Lays the framework for external volume mounts, preparing for 
persistent disks and other volumes.

Currently, it only supports host directory mounts.

Modifies the API to support host directory mounts from Volumes
instead of VolumeMounts.

We should capture today's default behavior as an Empty or EmptyDir or something.  The actual structure has no fields, but this makes the API absolutely explicit.