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

@@ -23,6 +23,10 @@ import uuid
 from distutils.version import LooseVersion as Version
 import dateutil.parser
 import yaml
+try:
+    import psutil
+except:
+    psutil = None
 
 # Pexpect
 import pexpect