avocado.utils.process: Fix system_output() call

Suppress debug output in this call, which is unnecessary.
Also, ignore its rc, given that we'll kill child processes
only if they actually exist.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #255 from lmr/process-bugfix

avocado.utils.process: Fix system_output() call