Cloning large repositories over the internet can take considerable
time. Caching them locally makes repeated image builds significantly
faster, so lets do that.
When users override the element source they will often be using a
local repository, so in those cases I don't cache - but we could
easily change our minds on that in the future.
Change-Id: I9822362cf722b904d9806dbbb4bb07cfe2b33437
Git isn't a dependency of this element, but is a dependency of pbr
if you are doing pip install on a git cloned directoy, because this
looks to be a fairly common pattern it makes sense to have git on the
target image.
Change-Id: I49ee3fa4eac493b9dfa379a674ab9b3390cfc89a
Fixes bug 1184943
Adding an element that allows other elements to register
repositories it would like to be retrieved for it.
Doing this outside of the chroot allows locally cached
repositories to be used. It also gives the ability to d-i-b
to specify what revision to use if an alternative to the
most recent is required.
Effectively allowing a CI system to test d-i-b and elements
without being effected by unavailable git repositories or
breakages in actively developed source code being used by
the elements.
Change-Id: I1527facebaad256a357af680e017b34b1788575d
