Allow Kinesis Endpoint to be Overridden via Config

I perform local testing of my application stack with Flink configured as a consumer on a Kinesis stream provided by Kinesalite, an implementation of Kinesis built on LevelDB. This requires me to override the AWS endpoint to refer to my local Kinesalite server rather than reference the real AWS endpoint. I'd like to add a configuration property to the Kinesis streaming connector that allows the AWS endpoint to be specified explicitly.

This should be a fairly small change and provide a lot of flexibility to people looking to integrate Flink with Kinesis in a non-production setup.