Set timetable for removal of oslo.messaging.notify.drivers

Set timetable for removal of oslo.messaging.notify.drivers

Icehouse is dead and gone, at least upstream. These special driver
registrations are not tested in the gate-tempest-dsvm-neutron-full job
which means they are also not tested in requirements constraints jobs.

oslo.messaging 2.6.0 broke these already by removing the internal modules,
which was fixed in o.m 3.0.0 with (deprecated) alias modules.

The minimum required version of o.m in mitaka is currently greater than
2.6.1, so we're OK to remove these once stable/mitaka is our oldest
supported branch. So add a TODO to remove these once liberty-eol happens.

Proper configuration for notification drivers happens through the
config file using the oslo_messaging options:

http://docs.openstack.org/developer/oslo.messaging/opts.html

Adds a release note for the deprecation and timetable.

Change-Id: I2c04117b0f531fe3604e74401d0ffd8d8d3dd308
Related-Bug: #1513630