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.

one of teh Go dev team made this same change (foo.FooInterface -> foo.Interface) in another module a few days ago.  I raised the same objection and was told This is standard Go.