Two different names for enabling tracing via environment variable

[Andy Shinn]

Documentation Page

• documentation/content/en/serverless/installation/python.md

  Line 242 in b7e7d4b

       - Set the environment variable `DD_TRACE_ENABLED` to `true`. 

• documentation/content/en/serverless/distributed_tracing/_index.md

  Line 41 in 160886e

   The Datadog Python, Node.js, Ruby, Go, Java, and .NET tracing libraries support distributed tracing for AWS Lambda. You can install the tracer using the [installation instructions][5]. If you already have the extension installed, ensure that the environment variable `DD_ENABLE_TRACING` is set to `true`. 

Issue Description

These two documentation pages seem to have two different environment variables for enabling tracing. One is DD_TRACE_ENABLED and the other is DD_ENABLE_TRACING. I am assuming only one is valid. Which one is correct?