workspaces.yaml has no versioning mechanism

!271 (closed) was submitted to add a versioning mechanism to workspaces.yaml so that after upgrading buildstream to a new version, it can cope with having older workspaces.yaml files.

Review suggests that the implementation in !271 (closed) may be overcomplicated, the changes in the wrong place and not tested at the correct level, so an alternative implementation is required to solve this issue, hence this issue has been created rather than hijacking the merge request.