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.

if you really want to be polite you would support this for a while.  "Deprecated" means "works but discouraged" not "suddenly broken".  Probably not a big deal, but might want to sync with @vmarmol