Feature: detect changes to credentials and reload

[rogermyung]

I have followed the directions at https://docs.aws.amazon.com/codedeploy/latest/userguide/register-on-premises-instance-iam-user-arn.html to generate temporary IAM credentials and push them to the deployment server. However, the agent doesn't reload the credentials unless I create a separate service that watches for a changed file and restarts the agent. I was directed to this by AWS support. This process results in a delay in every deployment while we wait for the agent to start back up.

We would prefer that the agent detects changes to the conf yml file and reloads as needed.