Fix coverage reports

Fix coverage reports

Currently, the coverage reports include the unittests themselves, and
this skewes unfairly the reports, as the coverage for the tests is very
high (since they all run).

To fix this, we export the ganeti temp dir from run-in-temp-dir, and we
use that to exclude the tests directory. The patch also fixes a but
related to multiple directories to be omitted (--omit a --omit b is
wrong, it needs to be --omit a,b).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>