Add ability to configure dhcp_agent_notification

Add a new dhcp_agent_notification parameter to allow its configuration.

Installation without any DHCP agents should have this option disabled
to avoid these kinds of error messages in Neutron logs:

- No more DHCP agents
- Unable to schedule network XXX: no agents available;
  will retry on subsequent port creation events.
- Will not send event port_create_end for network XXX:
  no agent available.

Change-Id: I42a5a045b777e42e5c99cea16a0fbc6ed145d73b
Merge "Add ability to configure dhcp_agent_notification"