Add volume metadata to v2

This exposes volume metadata in the Cinder v2 API. This already exists
in v1 and was originally suppose to exist back with Grizzly, but appears
to have slipped.

Closes-Bug: #1227979
Change-Id: I512dad591d7d491eca54a230d3cc290d9a349e6f
Merge "Add volume metadata to v2"