Bug 1591766 unveiled an issue where calling the plugin API does not trigger
DHCP notifications. This is required by the auto-allocated-topology service
plugin that calls core_plugin.update_network(), and expect notifications
to be sent out on state changes. To accomplish this, the logic has been
encapsulated in the DHCP module, and leveraged via callback mechanisms.
For this reason, new events have been introduced, AFTER_REQUEST, and
BEFORE_RESPONSE. The latter in particular is the one needed to hook up
dhcp notifications in order to preserve backward compatibility.
More precisely, core plugins that use DHCP as is or implement their own,
(with or without an agent) should already instantiate their own notifier,
and if they do not, this should be rectified.
A search on codesearch.openstack.org reveals that out-of-tree plugins
already specify their own notifiers, and the default initialization is
clearly redundant now.
Related-bug: #1591766
Change-Id: I7440becb6d30af7159ecaeba09d7a28eceb71bea
877778ee4c