The ubuntu element supports simple replacement of sources.list by using
the DIB_DISTRIBUTION_MIRROR environment variable. Drag the debian
element into line by supporting both DIB_DEBIAN_MIRROR and the new shiny
DIB_DISTRIBUTION_MIRROR.
Change-Id: I9d5f8d1e3251965b34e55929182aa601a524fe8f
Also allows setting the "distro name" so that things that change the
debootstrap signature will have a different tarball.
This is being done in advance of a debian-upstart element to allow
experimenting with debian and upstart.
Change-Id: I1a558fdaf5924c65f1cced9f9bf29d73643c30af
