Create notification mechanisms

[congwang09]

Create notification mechanisms in SDX controller, that sends notifications to a separate RabbitMQ (create a new queue dedicated for notifications). The message can include VLAN number and event.
SDX controller will not care whom the messages will be sent to. It's up to the users to subscribe to the queue.

We should define what events should be notified, maybe: path re-computation, configuration change, reprovisioning, VLAN removal, or everything that affects the service to be notified.

@jab1982 Create a blueprint on what types of notification should be included.

[YufengXin]

I can implement this using gmail server, likely straightforward with the implemented state machine.

If you like gmail, what email address do you like: sdx-ops@gmail.com? @jab1982 @italovalcy

I think gmail password authentication is enough.
GMAIL API + OAuth 2.0 is probably an overkill, and it requires a credit card though the service is free.