Use except x as y instead of except x, y

According to https://docs.python.org/3/howto/pyporting.html the
syntax changed in Python 3.x. The new syntax is usable with
Python >= 2.6 and should be preferred to be compatible with Python3.

Change-Id: I4a951aecc32ef9e5131da236ae4815b84897d67b

tests/installer/test_utils.py

@@ -86,7 +86,7 @@ class ParameterTestCase(PackstackTestCaseMixin, TestCase):
             execute('echo "mask the password" && exit 1',
                     use_shell=True, mask_list=['password'])
             raise AssertionError('Masked execution failed.')
-        except ExecuteRuntimeError, ex:
+        except ExecuteRuntimeError as ex:
             should_be = ('Failed to execute command, stdout: mask the %s\n\n'
                          'stderr: ' % STR_MASK)
             self.assertEqual(str(ex), should_be)