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.

Further thoughts:

It doesn't matter right now, since your HostDirectory volume is totally idempotent and doesn't produce any state. But as soon as an implementation that keeps state shows up, we'll need to address the issue where every re-invocation of a container produces a new mounting of a volume. Because of this issue, I think you should consider coming up with a deterministic temporary directory scheme. You could potentially use the container manifest id and the volume name (?).