Adds Python 3.x compatible exception handling

Fixes exception handling syntax for Python 3.x compatibility

Changes generated with:

    2to3 -f except -w -n cloudbaseinit

cloudbaseinit/metadata/factory.py

@@ -47,7 +47,7 @@ def get_metadata_service():
         try:
             if service.load():
                 return service
-        except Exception, ex:
+        except Exception as ex:
             LOG.error("Failed to load metadata service '%s'" % class_path)
             LOG.exception(ex)
     raise Exception("No available service found")