swift-ring: manage swift rings via git

first attempt at being able to code-review changes to the swift rings,
currently this taps into our normal workflow of distributing the ring files via
the puppet master. That might change in the future and we can consider using
trebuchet for that task too in addition to this.

TODO:
* the puppet master name is hard coded to palladium

Change-Id: I69ce0852586b364e456cca7a6035db8cbf55c2f9