classes: add class for retaining build results

classes: add class for retaining build results

If you are running your builds inside an environment where you don't
have access to the build tree (e.g. an autobuilder where you can only
download final artifacts such as images), then debugging build failures
can be difficult - you can't examine log files, the source tree or
output files. When enabled, this class does two things:

1) Triggers on task failure and saves a tarball of the work directory
   for the task's recipe
2) Optionally saves tarballs of a list of nominated directories

It puts these tarballs in a configurable location, where they can be
picked up by a separate process and made available as downloadable
artifacts.

Signed-off-by: Paul Eggleton <paul.eggleton@microsoft.com>