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.

Are there examples in the stdlib?  I ask because that surprised me.  I know effective go encourages verbing the noun even if "Volumer" sounds horrible.