Add endpoint to config for testing and development

[dennyjun]

I realized there's no way to set the endpoint if I am running Kinesis locally for testing and development. I couldn't get the local version of logstash-input-kinesis to run so I couldn't test it but code should work. If I can get a link to getting custom plugins to run locally that actually works I would really appreciate it!

[codekitchen]

I typically follow a process similar to the one outlined here for local testing: https://www.elastic.co/guide/en/logstash/current/_how_to_write_a_logstash_output_plugin.html#_building_and_testing_4

Basically, gem build the plugin, then point logstash-plugin install to that local .gem file.

This change should allow using a local kinesis, but it will still require a real AWS dynamodb table. We should probably add an option to configure the dynamo endpoint as well, so that you can use local dynamo.