torusctl: support create volume from another volume's snapshot

[nak3]

fixes #273

[nak3]

usage example

1. prepare snapshot

./torusctl block snapshot list myvol
Volume: myvol
SNAPSHOT NAME  TIMESTAMP
snap1          2016-11-22T02:07:29Z
snap2          2016-11-22T02:07:30Z

2. create volume from another volume's snapshot

./torusctl volume create-block from-snapshot myvol@snap2 newvol --progress
newvol [==========================================================]  100 MB / 100 MB

3. new volume is created from another volume

./torusctl volume list
VOLUME NAME  SIZE    TYPE
newvol       95 MiB  block
myvol        95 MiB  block

[lpabon]

This should not be done in the command line code. Command line code is supposed to send an RPC request to the service. This way future calls from other clients can do this also.

[mischief]

i'd love to see this happen - but it does need to be done server side.

[nak3]

I have already moved the code to block package and it could address the concern of command line code. I'm sorry, but could you please elaborate on what you mean the "server side"?

[joshsouza]

This feature would be incredibly valuable for my team's use-case. We are looking into alternatives such as Ceph or Portworx to create new persistent storage volumes for containers from a base snapshot, but this would make Torus a competitive solution. Any way we can get eyes on this?