issue #625: use exec() instead of subprocess in mitogen_ansible_playbook

This is just to make CTRL+C handling less confusing. Alternate would be
ignoring SIGINT, but this is simpler.

tests/ansible/mitogen_ansible_playbook.py

@@ -3,4 +3,8 @@ import os
 import subprocess
 import sys
 os.environ['ANSIBLE_STRATEGY'] = 'mitogen_linear'
-subprocess.check_call(['./run_ansible_playbook.py'] + sys.argv[1:])
+os.execlp(
+    './run_ansible_playbook.py',
+    './run_ansible_playbook.py',
+    *sys.argv[1:]
+)