Send DHCP notifications regardless of agent status

The Neutron service, when under load, may not be able to process
agent heartbeats in a timely fashion.  This can result in
agents being erroneously considered inactive.  Previously, DHCP
notifications for which active agents could not be found were
silently dropped.  This change ensures that notifications for
a given network are sent to agents even if those agents do not
appear to be active.

Additionally, if no enabled dhcp agents can be found for a given
network, an error will be logged.  Raising an exception might be
preferable, but has such a large testing impact that it will be
submitted as a separate patch if deemed necessary.

Closes-bug: #1192381
(cherry picked from commit 522f9f94681de5903422cfde11b93f5c0e71e532)

Change-Id: Id3e639d9cf3d16708fd66a4baebd3fbeeed3dde8
Merge "Send DHCP notifications regardless of agent status" into stable/havana