Periodically retrieve public IP from EC2 metadata and update DNS records.

EC2 Instance Metadata http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-instance-metadata.html#instancedata-data-retrieval

Rancher host label io.rancher.host.external_dns_ip http://docs.rancher.com/rancher/latest/en/rancher-services/dns-service/#using-a-specific-ip-for-external-dns

• RANCHER_URL = Rancher Server URL
• RANCHER_KEY = Environment API Access Key
• RANCHER_SECRET = Environment API Key Secret
• RANCHER_DNS_SERVICE = Service name of External DNS Service
• PERIOD = default is 10 minutes (e.g. '10m')
• RAYGUN_APIKEY = Raygun API Key