Avoid using os.system since it serializes threads on macosx.

* Use subprocess.Popen instead.

  See http://docs.python.org/library/subprocess.html#replacing-os-system.

Configuration/PyReleaseValidation/python/WorkFlowRunner.py

@@ -3,6 +3,7 @@
 
 from Configuration.PyReleaseValidation import WorkFlow
 import os,time
+from subprocess import Popen 
 
 class WorkFlowRunner(Thread):
     def __init__(self, wf):
@@ -30,7 +31,8 @@ def doCmd(self, cmd, dryRun=False):
 
         ret = 0
         if not dryRun:
-            ret = os.system(cmd)
+            p = Popen(cmd, shell=True)
+            ret = os.waitpid(p.pid, 0)[1]
             if ret != 0:
                 print "ERROR executing ",cmd,'ret=', ret