Add support for git tracked metadata.

This is intended to let other people test drive using git tracked
metadata.  Support is not complete -- the dev tool still needs to be
updated to pull in and track the metadata tracking repo like it tracks
the barclamps, and support needs to be added to ./dev setup to start
pulling in the metadata tracking repo.
Merge pull request #1951 from VictorLowther/support-git-tracked-metadata

https://github.com/crowbar/crowbar/pull/1951

Merged by devtool for VictorLowther