Add root, tmp and data_dir to cartesian config

Those variables can be used in cartesian config by ${autotest_*_dir}.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
qemu.tests: Use $autotest_tmp_dir instead of /tmp/

Use $autotest_tmp_dir instead of hardcoded /tmp/ for tests
which creates large files to avoid filling of memory (as
the latest dirstribution use tmpfs for /tmp).

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>