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 and empty directories.

Modifies the API to support mounts from Volumes
instead of VolumeMounts.

Nope, not without failing the request and sending back an error. Now that I think about it more, though, there's nothing stopping us from additionally running the validation client-side, where warnings *would* make sense. We should do that anyway. So, yeah, leave the warnings in, I guess?