incorporate kernel team's adt-matrix for per-kernel-flavor test attribution

[Steve Langasek]

Ubuntu's britney has hard-coded logic to avoid blocking kernel migrations due to autopkgtest regressions, because some tests may pass or fail on a per-kernel-flavor basis and it becomes unmanageable (due to release team bottleneck) to unblock these kernels.

The kernel team therefore does analysis of the regressions external to britney, and manually unblocks kernels for migration when they've confirmed that there are no regressions.

report here: http://people.canonical.com/~kernel/status/adt-matrix/
code here: lp:~canonical-kernel/+git/kteam-tools master adt-matrix

Unfortunately this process is then entirely opaque to the release team or other consumers of update_excuses.

This logic should be internalized into britney so that the kernel team doesn't have to bolt it onto the side.