[FLINK-3270] Add Kafka example

[rmetzger]

No description provided.

[StephanEwen]

Not quite sure about adding examples for the connectors to the examples project. If we do that for all connectors, it becomes heavily loaded with dependencies.

Can the example not be in the kafka connector project?

[rmetzger]

I understand that argument, and I would be strongly against adding examples for each connector.

The pull request is moving the example away from the Kafka project into the main Flink examples.
The reasons why I wanted to have Kafka in the main examples:

• Almost all Flink users I know are using Kafka, so its important to place the example prominently.
• Its very handy to deploy a Kafka example with the binary distribution to quickly test if flink is properly working on a cluster with Kafka (instead of building a separate project somewhere)

[fhueske]

I think Robert has a good point.
The examples should follow the implementations of the other examples after the rework in PR #1581.