code: function get_client_registration(client_id) {
  
      return new Promise((resolve, reject) => {
          if (client_id === "123456") {
              return resolve({
                  redirect_uri_hostname: "localhost.sampleapp.com",
                  redirect_uri_port: "8443",
                  redirect_uri_path: "/sample_app/client_services",
                  description: "sample app",
                  client_id: client_id,
                  username: "test_client@clientcorp.com",
                  password: "client_pwd"
  
              });
          } else {
              return reject(new Error("client ID not registered!"));
          }
      });
  }
docs: This method must retrun a Promise which resolves to the client registration
  entry from a persistent store.  The client registration entry must have attributes
  identical to the ones in this demo implementation (redirect_uri_hostname,
  redirect_uri_port, redirect_uri_path, and description).
  
  This is a demo implementaiton of the function implementing client registry
  lookup.  It has a hard-coded entry for the sample app running on:
  https://localhost:8443
  with client_id: 123456
  
  For demo purposes, the http basic auth header is:
  authorization: Basic MTIzNDU2OmNsaWVudF9wd2Q=
  
  @param  {[type]} client_id
  @return {[type]}