p-s: use upgrade-test-distros to select the test

use len(get_distros()) > 1 to enable the distupgrade test

drop support for running both install/purge and distupgrade test for a section

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p-s: simplify piuparts command generation

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p-s: make test_package() a member function of Section

simplify access to config and distro_config

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p-s: _test_package(): use self._config

instead of passing a config parameter

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p-s: simplify package existence checks

Signed-off-by: Andreas Beckmann <anbe@debian.org>
lib/conf: add get_basetgz() method

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p-s: add basetgz-directory setting and _get_tarball() method

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p-s: use the _get_tarball() method

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p-s: declare upgrade-test-chroot-tgz obsolete

Signed-off-by: Andreas Beckmann <anbe@debian.org>
Merge remote-tracking branch 'andreas/feature/no-redundant-config' into develop
add note that this suggestion might not be the best..
piatti.conf: priorize squeeze2bpo2wheezy higher