Adjust psutil usage for psutil > 2

psutil 2.x and above has a lot of API changes as described in:

https://github.com/giampaolo/psutil/blob/master/HISTORY.rst

Change-Id: I1380f488390c83a44d91b84218f1b1ba826d03a0
Closes-Bug: 1659146

neutron/tests/functional/test_server.py

@@ -110,7 +110,10 @@ class TestNeutronServer(base.BaseTestCase):
 
         def safe_ppid(proc):
             try:
-                return proc.ppid
+                if psutil.version_info[0] == 1:
+                    return proc.ppid
+                else:
+                    return proc.ppid()
             except psutil.NoSuchProcess:
                 return None