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.

Seems we need to make Source a pointer to actually check for uninitialized structs in Go. Is there a better/another way to do this?