Decouple backend exec from python

Current backends (pox, ryu) are python-based, so executing them through
python works, but it ties the frontend and backend to the same python
version. This change should make it easier to move pyretic to python3
independent of the backend python version.

Author: JamesGuthrie

pyretic.py

@@ -197,22 +197,28 @@ def log_writer(queue, log_level):
             if backend_path is None:
                 print 'Error: {} not found in PYTHONPATH'.format(backend_client)
                 sys.exit(1)
-            backend_exec = os.path.join(backend_path,'pox.py')
+            # Pox is still py2-only
+            if sys.version_info[0] == 2:
+                python = sys.executable
+            else:
+                python = which("python2")
+                if not python:
+                    print "Error: Could not find 'python2' in path."
+                    sys.exit(1)
+            backend_exec = [python, os.path.join(backend_path, 'pox.py'), backend_client]
         elif options.backend == 'ryu':
             backend_client = 'of_client.ryu_shim'
-            backend_exec = which('ryu-manager')
-            if not backend_exec:
+            ryu_exec = which('ryu-manager')
+            if not ryu_exec:
                 print "Error: Could not find 'ryu-manager' in path. Is ryu installed?"
+            backend_exec = [ryu_exec, backend_client]
         else:
             print "Error: Invalid backend '{}' specified".format(options.backend)
             sys.exit(1)
 
-        python=sys.executable
         # TODO(josh): pipe backend stdout to subprocess.PIPE or
         # other log file descriptor if necessary
-        of_client = subprocess.Popen([python,
-                                      backend_exec,
-                                      backend_client ],
+        of_client = subprocess.Popen(backend_exec,
                                      stdout=sys.stdout,
                                      stderr=subprocess.STDOUT)