Removed direct access to MessagingServer

Factory method provided by the library should be used to create a server
object. This allows to change servers internal implementation without
touching end users.

Also removed custom dispatcher. Its only purpose was to log messages.
If logging is configured accordingly all incoming messages will be
logged from oslo/messaging/_drivers/amqpdriver.py [1]

[1] http://goo.gl/nV9tcu

Closes-Bug: #1357236

Change-Id: Ic208994c5a64fd48528cb41d30a975d68d84af05
Merge "Removed direct access to MessagingServer"