use aws ec2 metadata for region config

[trjstewart]

Is your feature request related to a problem? Please describe.
I am currently planning a globally HA NGINX+ deployment and with the configuration options as they are currently it would mean we have to bake a separate AMI for each region, with this config being the only static portion of it.

Describe the solution you'd like
I would like to add an additional self option to the region configuration that uses the EC2 Metadata service to retrieve the current instance region.

region: self
api_endpoint: http://127.0.0.1/api
sync_interval_in_seconds: 1
cloud_provider: AWS
upstreams:
 - name: nginx-poc-api
   autoscaling_group: nginx-poc-api
   port: 3000
   kind: http

Describe alternatives you've considered
I considered removing the region configuration all together and only using the EC2 Metadata service for configuration but I definitely think there is a use-case to allow for cross-region upstreams.