Improve Kafka support for device notifications (from device manager) and device updates (from the device itself)

[giovannicuriel]

The current Kafka support is nice, but it can be improved.

Currently, there are two "communication sides":

• Receiving device notifications from device manager: this is when iotagent gets to know all devices whose messages should be processed by it.
• Sending device updates to all components interested in them: when the physical device sends a message to iotagent, it processes it and send a nicer version of this message to other components. Separating device CRUD notifications from physical device updates #16 allows this update to be sent through Kafka or through Orion.

There are a bunch of configurable things in these two scenarios, but I don't think they are in the same page. For instance, I can configure which topic will be used to receive the device notifications, its maximum read buffer, and so on, but from the device update side there is no configuration whatsoever.