Clean error.message

Unfortunately the way that jsonschema is calling %r on its property
and then encoding the complete message means I've had to do this
manual way of removing the literal string prefix, u'.

eg:

key = 'extends'
message = "Invalid value for %r" % key
error.message = message.encode("utf-8")"

results in:

"Invalid value for u'extends'"

Performing a replace to strip out the extra "u'", does not change the
encoding of the string, it is at this point the character u followed
by a '.

Signed-off-by: Mazz Mosley <mazz@houseofmnowster.com>

tests/unit/config_test.py

@@ -659,7 +659,7 @@ class ExtendsTest(unittest.TestCase):
             )
 
     def test_extends_validation_missing_service_key(self):
-        with self.assertRaisesRegexp(ConfigurationError, "u'service' is a required property"):
+        with self.assertRaisesRegexp(ConfigurationError, "'service' is a required property"):
             config.load(
                 config.ConfigDetails(
                     {