osmoutil: open /dev/null only once

Change-Id: Ic6da34f6c4c5cd3b0786788f4e38c9c6248fca1b

osmopy/osmoutil.py

@@ -23,9 +23,14 @@ import time
 
 """Run a command, with stdout and stderr directed to devnull"""
 
+devnull = None
 
 def popen_devnull(cmd, verbose=True):
-    devnull = open(os.devnull, 'w')
+    global devnull
+    if devnull is None:
+        if verbose:
+            print "Opening /dev/null"
+        devnull = open(os.devnull, 'w')
     if verbose:
         print "Launching: PWD=%s %s" % (os.getcwd(), ' '.join([repr(c) for c in cmd]))
     return subprocess.Popen(cmd, stdout=devnull, stderr=devnull)