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.

My fear is that it might exist but not be properly prepared.  Even simpler yet - why not just say that SetUp has to be idempotent, and don't bother checking at all.