cfg: Fix tcpdump kill command to work on a wider variety of hosts

Using 'pkill -9 tcpdump' works on both new and older
distros.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #1270 from autotest/fix-tcpdump-kill

cfg: Fix tcpdump kill command to work on a wider variety of hosts