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.

SetUp and TearDown may perform procedures in addition to mounting/unmounting depending on the volume. I'll edit the comments to make this a bit more clear.