Use Python 3.x compatible except construct

Per (proposed) Hacking check, use except x as y:
instead of x,y:, which works with any Python version >= 2.6

Change-Id: I6e52cedec5c694bccae10541b1964a8547d4ffd1

ceilometerclient/shell.py

@@ -335,7 +335,7 @@ def main():
     try:
         CeilometerShell().main(sys.argv[1:])
 
-    except Exception, e:
+    except Exception as e:
         print >> sys.stderr, e
         sys.exit(1)