Mark the ZeroMQ driver deprecated

As per the Dublin 2018 PTG decision:

http://lists.openstack.org/pipermail/openstack-dev/2018-March/128055.html

Change-Id: I1cda5dffbc29aad5fea001a79562db7c144a339b

doc/source/admin/zmq_driver.rst

@@ -8,6 +8,12 @@ ZeroMQ Driver Deployment Guide
 Introduction
 ============
 
+**Note:** The ZeroMQ driver has been **deprecated** and is no longer
+maintained.  Refer to the mailing list announcement for more
+`details`_.
+
+.. _details: http://lists.openstack.org/pipermail/openstack-dev/2018-March/128055.html
+
 0MQ (also known as ZeroMQ or zmq) is embeddable networking library
 but acts like a concurrency framework. It gives you sockets
 that carry atomic messages across various transports

oslo_messaging/_drivers/impl_zmq.py

@@ -15,6 +15,7 @@
 import os
 import threading
 
+from debtcollector import removals
 from stevedore import driver
 
 from oslo_messaging._drivers import base
@@ -60,6 +61,8 @@ class LazyDriverItem(object):
             self.item.cleanup()
 
 
+@removals.removed_class('ZmqDriver', version='Rocky', removal_version='Stein',
+                        message='The ZeroMQ driver is no longer supported')
 class ZmqDriver(base.BaseDriver):
 
     """ZeroMQ Driver implementation.