title: Environment variables in Envoy config
body: <p>How could i use environment variable in the envoy-config.xml . I am deploying envoy using the docker image.
  for example in below case i want to change the port number (EDGE_ENVOY_ADMIN_PORT) which is defined in my .env file</p>
  
  <pre><code>admin:
    access_log_path: "/opt/edge-envoy/logs/admin_access.log"
    address:
      socket_address:
        address: "0.0.0.0"
        port_value:   ${EDGE_ENVOY_ADMIN_PORT}
  </code></pre>
  
  <p>is there any way such token replacement functionality available in envoy? </p>