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.

Is this is Host directory in the sense of the REAL host, or in the sense of what the default Volume struct is today.  Comment on this should be clearer.