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.

If I understand, this is a cache of mounted volumes, right?  That way you can de-dup singleton resources like PDs or whatnot.  But most "kinds" of volumes don't need such a cache.  I suggest making the cache be smaller in scope - closer to the actual use-case for it.  For simple directories, I don't see a need to cache at all, but I will read-on and see if I figure out something different :)