zephyr_mirror: Use SIGTERM to try to kill children.

It's generally best to first try killing with SIGTERM, and if we fail
to kill the child, we'll end up trying to kill them with SIGKILL
anyway when the new zephyr_mirror process starts up,

(imported from commit cfee2dd5f809f6e38d90a09be82719a8660d8377)

bots/zephyr_mirror.py

@@ -205,7 +205,7 @@ def maybe_restart_mirroring_script():
         print
         print "%s: zephyr mirroring script has been updated; restarting..." % \
             (datetime.datetime.now())
-        os.kill(child_pid, signal.SIGKILL)
+        os.kill(child_pid, signal.SIGTERM)
         while True:
             try:
                 if bot_name == "extra_mirror.py":