Expose volume potential

[wimpers]

Continuation of openvstorage/openvstorage-health-check#293

• Expose the volume potential in the api
• Adjust vDisks creation, cloning, move vdisk so it takes the volume potential into account.

[khenderick]

Exposing it over the API is fairly easy, but keep in mind that blocking certain actions (move, creation, ...) can still fail due to race conditions (we check the potential which is e.g. 1, and by the time the move is executed some other volume was moved so it fails anyway)

[khenderick]

We might also only add some kind of warning-ish thing when a volume is moved to a node that is low on capacity. We might also always show this number in the GUI for example.

[wimpers]

Discussion from the BAM: show warning and expose as API

[matthiasdeblock]

Can be useful as well on the Nuvolat setup as India want this in their SIMF to check before deploying volumes.

[matthiasdeblock]

Moving a volume to a storagerouter with 0 volume potential should result in a warning/error without moving the volume to the new storagerouter which cannot start the volume as the potential is 0.

Currently the volume is getting moved which results in a non-running volume.

[jeroenmaelbrancke]

@wimpers
When will this be implemented in andes-updates-4 and bighorn-rc-2?
Nuvolat struggles with this as they don't know where to deploy the vdisks.
SIMF deploys vdisks on random machines without the knowledge if there is space enough or not.

[wimpers]

Back to BAM.

[JeffreyDevloo]

Partially fixed by #2151
Packaged in openvstorage-core_2.9.20-1_amd64.deb
Released in https://github.com/openvstorage/framework/releases/tag/2.9.20 and https://github.com/openvstorage/framework/releases/tag/2.13.0