Exporter does not respect xray daemon

[emman27]

According to AWS X-Ray documentation, one possible deployment method is to use a X-Ray daemon on a Kubernetes cluster to avoid provisioning permissions unnecessarily to applications to write to X-Ray.

The X-Ray SDK from AWS respects the following environment variables to do service discovery of the daemon. Right now, this exporter ignores this environment variable and bypasses by making direct API calls to X-Ray.

I propose to have the opencensus exporter respect this environment variable to match the behavior in the AWS documentation, making setup simpler and more configurable.

Reference documentation:
https://docs.aws.amazon.com/xray/latest/devguide/xray-sdk-go-configuration.html#xray-sdk-go-configuration-envvars
https://aws.amazon.com/blogs/compute/application-tracing-on-kubernetes-with-aws-x-ray/