lib/conf: add get_distros() method

returns the split upgrade-test-distros list

Signed-off-by: Andreas Beckmann <anbe@debian.org>
lib/conf: use get_distros()

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

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p-s: use the new get_distros() and get_start_distro() methods

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p-r: generate default descriptions

two different descriptions depending on the test: install/purge or
distupgrade

the description setting in piuparts.conf can be omitted to use the
default description

if the description setting in piuparts.conf starts/ends with a '+',
it will be appended/prepended to the default description

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p.conf: use generated descriptions

Signed-off-by: Andreas Beckmann <anbe@debian.org>