Most of our distros support DIB_DISTRIBUTION_MIRROR for setting the
mirror to be used when building. This has a major downside where if
someone would like to create a static configuration for building various
distro images it cannot set DIB_DISTRIBUTION_MIRROR (think of the case
where we want to run our tests).
By adding support for DIB_$DISTRO_DISTRIBUTION_MIRROR this problem is
resolved.
Change-Id: I92964b17ec3e47cf97e3a3091f054b2a205ac768
cd1965f7b7