This patch solves three issues with Debian packaging / apt:
o When building 'testing' only default apt sources is
included - backports, updates and security are skipped because they
do not exists.
o The default release for Debian was `unstable`: this is now fixed to
`stable`.
o Starting a Debian Stretch VM that was build with diskimage-builder
does not work, because some mandatory packages are missing.
This patch fixes this problem: it adds the mandatory packages and
the test case.
Change-Id: If49b5b162c4da1e074e9b19324839bc59d87dc57
Signed-off-by: Andreas Florath <andreas@florath.net>
