Fix psutil usage on el6 for job cancel

psutil will fail to import on el6 due to not being able to access a
pseudo terminal.  This issues a SIGKILL to the proot process in order to
force the stop

awx/main/tasks.py

@@ -23,7 +23,10 @@ import uuid
 from distutils.version import LooseVersion as Version
 import dateutil.parser
 import yaml
-import psutil
+try:
+    import psutil
+except:
+    psutil = None
 
 # Pexpect
 import pexpect
@@ -434,6 +437,9 @@ class BaseTask(Task):
             if instance.cancel_flag:
                 try:
                     if settings.AWX_PROOT_ENABLED:
+                        if not psutil:
+                            os.kill(child.pid, signal.SIGKILL)
+                        else:
                             main_proc = psutil.Process(pid=child.pid)
                             if hasattr(main_proc, "children"):
                                 child_procs = main_proc.children(recursive=False)