Fix no message attribute in exception

For py35, message attribute in exception seems removed.
We should directly get the string message from exception object
if message attribute not presented. And since get message attribute
already been deprecated. We should remove sopport on
exception.message after we fully jump to py35.
Partial-Bug: #1704725

Change-Id: I3970aa7c161aa82d179779f1a2f46405d5b0dddb

heat/common/pluginutils.py

@@ -12,6 +12,7 @@
 #    under the License.
 
 from oslo_log import log as logging
+import six
 
 
 LOG = logging.getLogger(__name__)
@@ -21,5 +22,6 @@ def log_fail_msg(manager, entrypoint, exception):
     LOG.warning('Encountered exception while loading %(module_name)s: '
                 '"%(message)s". Not using %(name)s.',
                 {'module_name': entrypoint.module_name,
-                 'message': exception.message,
+                 'message': getattr(exception, 'message',
+                                    six.text_type(exception)),
                  'name': entrypoint.name})