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.

Added a bool to the function. Since we can guarantee a unique directory path for every valid volume specification based on the name and podID (other than host directories, which this doesn't matter for), I think checking for the existence of the directory is sufficient for now.