Ensure get_pid_to_kill works with rootwrap script

To ensure that correct process is killed when using a rootwrap
script, we must recursively list the children of our top-level
process and kill the last one. This patch uses the psutil python
module which is already used in the heat-cfntools project.

Change-Id: I702bb9dd794c08fcaab637284ee303de1778cbb9