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.

Comment: This is optional for now.  If not specified, the Volume is implied to be an EmptyDir.  This implied behavior is deprecated and will be removed in a future version.