Switch from aptitude to apt-get as package manager in package list

So far we used "PACKAGES aptitude" to use aptitude as package
manager. FAI's install_packages supports different commands
(package managers) though, see `install_packages -H`:

  % install_packages -H | grep -e '^\s*aptitude ' -e '^\s*install '
         aptitude    aptitude -R -y -o Dpkg::Options::=--force-confdef -o Dpkg::Options::=--force-confnew install
          install    apt-get -y -o Dpkg::Options::=--force-confdef -o Dpkg::Options::=--force-confnew --fix-missing install

FAI versions until 5.3** support ignoring packages via:

| PACKAGES install
|
| packagename-

We can use that feature in the IGNORE class to avoid e.g. the
installation of the partimage package which is currently
unavailable in Debian/testing and therefore would fail the build
(because of aptitude's #835372 behavior change).

** NOTE: FAI v5.3 is broken regarding this behavior, so we've to
use FAI 4.3.1+deb8u1 from jessie to use that feature (at least
until it's restored/fixed again).