When cleaning up a running experiment, always attempt to kill

When cleaning up a running experiment, always attempt to kill
all processes in all vimages first.

It seems that quagga has a bug which sets all zebra processes in an
infinite loop if a network interface disappears while the suite is
running.  Before this change, we were "unplugging" the ng_eifaces
while zebra & co. were still running, resulting in each vimage
attempting to hog the CPU.  This would leave very little CPU time
for IMUNES scripts, and for any user interaction with the system
for that matter, so cleaning up a more complex topology could
last for quite a while.

In short, the cleanup should be now roughly as fast as it used to be
in 4.11 days.

Bug found by:
Submitted by:
Reviewed by:
Approved by:
Obtained from: