Lookup IOT Endpoint as part of deploy

[waynerobinson]

Thanks for the great library! :)

Not sure where best to put this, but I recently created a simple CloudFormation stack that has a single output, the IOT Endpoint that makes it easy to use in other stacks throughout your AWS account.

https://github.com/waynerobinson/aws-iot-endpoint-stack

This allows me to replace the awsIotHost definition with awsIotHost: ${cf:iot-endpoint.IotEndpointAddress} and as long as the stack is deployed in the account I'm using Chromeless in, it can refer to the correct IOT endpoint.

Happy to create a PR here, but not quite sure where you'd like this added. It's possible to just add this function to the serverless.yml file and have it called directly as part of the deploy, rather than relying on a pre-existing stack.