spec: Add tags to VolumeInfo

[akutz]

This patch adds an optional field tags to VolumeInfo. The tags field is a map[string]string and can contain additional attributes related to a volume.

Related to #116, #117. cc/ @saad-ali @jieyu

[saad-ali]

This looks mostly fine to me. It does complicate VolumeInfo in that it adds a field that should be populated on responses but should not be present on requests. Wondering if there is anything we could do to make that cleaner, but I do like having essentially a single object that captures all volume attributes. So I'm not sure.

[akutz]

Hi Saad,

I’d argue that the validate volume caps doesn’t need the volume info obj anyway, only the handle it contains. Why not just update the validate rpc to use the handle directly?

[saad-ali]

Good point. I would support replacing VolumeInfo with id in ValidateVolumeCapabilities along with this change.

[akutz]

Cool. I’ll update the PR with this change when I return to my desk.

[jdef]

Are tags stable over time, or do they have a TTL? How are these different than metadata, other than the fact that they're not passed back into the plugin for publish/validation calls?

[jdef]

There are no concrete use cases for this, so I'm closing it out. We can revisit later if needed. If you disagree, please re-open for comment.