Simplify public symbol exports

Inspired by tulip, have every module define a __all__ list and import *
from the top-level module.

Rename transport.set_defaults() since we don't want this to be a
top-level set_defaults() function as there may be multiple.

Also, rather than configuring flake8 to allow star imports, just exclude
the __init__.py files from flake8 checks.

oslo/messaging/exceptions.py

@@ -13,6 +13,8 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+__all__ = ['MessagingException', 'MessagingTimeout']
+
 
 class MessagingException(Exception):
     """Base class for exceptions."""