Support content source in create-volume

[aleofreddi]

Summary

One can use csc to create a snapshot, but there is no way to materialise back a snapshot into a volume because create-volume doesn't allow to specify the content source.

New Feature

The CSI specification allows you to specify the content source when creating a volume (a volume or a snapshot - see CreateVolume spec). This issue is to propose the addition of two flags (--source-volume and --source-snapshot) to the csc create-volume command to optionally fill in the content source.