Fixed audit notifications for l3-agent-router ops

Fixed audit notifications for l3-agent-router ops

l3-agent-router-add and l3-agent-router-remove do not
generate audit notifications which are used for security
compliance.
CRUD operations of core network resources are
handled by neutron/api/v2/base.py. In base.py, each of
create(), update(), delete() methods makes calls to
oslo.messaging.Notifier.info() to generate these notifications.
In the proposed fix, it is fixed in a similar fashion in
extensions/l3agentscheduler.py inside create() and delete()
methods by introducing info() method calls inside them.

Change-Id: I1354e51e4af24eda128c042926765683018b320b
Closes-Bug: 1317000