Adding element to get source for elements.

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