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.

I think this decision will help us later down the road when we want to add options for where we want to mount volumes (tmpfs vs disk,etc) and clarifies exactly what is needed to differentiate source types. I'll update the api to reflect this.