bugfix: kill the stunnel when cleanup

Popen.terminate() will kill the child process, but if you set the
shell argument to True, this is the process ID of the spawned shell,
so the stunnels are still exit.It will cause the failure of stunnel
setup in the next test runtime.

see: https://stackoverflow.com/questions/4789837/how-to-terminate-a-python-subprocess-launched-with-shell-true/4791612#4791612

Change-Id: I578611510f5bd1eb0dbd9a08ab408ccf101594f7
Signed-off-by: dongwenjuan <dong.wenjuan@zte.com.cn>