fix snapshot API response

[boddumanohar]

Fixes responses for snapshot API

• return created_at in RFC 3339 format
• In the pool-level listing volume_id is None, so None == model.lvol.get_id() is always False. meaning lvol would be None in every pool-level response even when the snapshot has a source volume.

[mxsrc]

I'm not sure I totally understand, a bit confused about the reasoning for passing the volume_id in in the first place. Shouldn't we just always use model.lvol.uuid?

[boddumanohar]

The SnapshotDTO is used at 2 places:

1. clusters:storage-pools:snapshots:list --> volume_id is empty
2. clusters:storage-pools:volumes:snapshots:list --> volume_id is passed. And that line checks if the passed in the volume_id is same as the current context which is in model.lvol.get_uuid()

At the moment, lvol is returned empty for pool level snapshots API. To prevent that, I added this extra check