Allow specifying default workspace in logan config

Usually we want the default workspace to be "development" but occasionally we have terraform configurations where the only active workspace is production. (E.g. https://gitlab.developers.cam.ac.uk/uis/devops/infra/gcp-infra/.) A recurring pain point is running terraform apply in that project and seeing terraform want to create 1000-odd resources before one remembers to specify --workspace=production.

The ability to have a default_workspace tag in the logan config would be much appreciated.