always run 'dpkg --configure -a' at the end of our dpkg callings

always run 'dpkg --configure -a' at the end of our dpkg callings

dpkg checks now for dependencies before running triggers, so that
packages can now end up in trigger states (especially those we are not
touching at all with our calls) after apt is done running.

The solution to this is trivial: Just tell dpkg to configure everything
after we have (supposely) configured everything already. In the worst
case this means dpkg will have to run a bunch of triggers, usually it
will just do nothing though.

The code to make this happen was already available, so we just flip a
config option here to cause it to be run. This way we can keep
pretending that triggers are an implementation detail of dpkg.
--triggers-only would supposely work as well, but --configure is more
robust in regards to future changes to dpkg and something we will
hopefully make use of in future versions anyway (as it was planed at the
time this and related options were implemented).

Note that dpkg currently has a workaround implemented to allow upgrades
to jessie to be clean, so that the test works before and after. Also
note that test (compared to the one in the bug) drops the await test as
its is considered a loop by dpkg now.

Closes: 769609