Fix metadata pre-populate logic

Related to gitlab-com/gl-infra/delivery#1763 (comment 582080434)

6ca4d2b0 - Don't upload metadata outside a coordinator pipeline

Previously this was a no-op because it was always empty outside these pipelines, but now if we pre-populate it, that won't be the case.

9a0b7401 - Fix metadata pre-populate logic

As we'll only want to do this on a real coordinator pipeline, we can't fetch the last release-tools tag, because that's the one we're running on. We actually need the previous tag.

The last method has been renamed to previous to reflect the change.

aa5d091d - Relocate metadata pre-population implementation

Because we're now pre-populating with data from the previous tag, the implementation was so specific to a tagged auto-deploy coordinator pipeline that it didn't make sense to be a generic method in the ReleaseMetadata class.

It's now entirely implemented in ReleaseTools::Tasks::AutoDeploy::Tag.