Fixed audit notifications for dhcp-agent-network

dhcp-agent-network-add and dhcp-agent-network-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/dhcpagentscheduler.py inside create() and delete()
methods by introducing info() method calls inside them.

Change-Id: Ia23b981b2fbe739d22ebaa7bb0975fb9e39f881b
Closes-Bug: 1317008
Merge "Fixed audit notifications for dhcp-agent-network"