osmoutil: centrally log process launches

Log Popen in osmoutil.py instead of in each python test. Launch message is
being removed in openbsc's python tests and added here. (change-id
Id8eb70ddfdc1d0d9f90aa5343a4ea522042c34ee)

Change-Id: I7e539abc3aeee91ae3e7635f9e28b9e557eaf969

osmopy/osmoutil.py

@@ -23,8 +23,10 @@ import importlib
 """Run a command, with stdout and stderr directed to devnull"""
 
 
-def popen_devnull(cmd):
+def popen_devnull(cmd, verbose=True):
     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)