title: Passing in template parameters
body: <p>We are looking into using aws-cdk in our CI/CD pipeline.  We need to be able to pass parameters into the the template during the build so it can generate an artifact to use during deploy.  I see we can use the cdk.json file to specify context properties, but this doesn't actually put the values into the CloudFormation template itself.  Just gives you access to them in the code.</p>
  
  <p>I have tried something like this:</p>
  
  <pre><code>const servicenameprop = new PipelinePrerequisitesProps();
  servicenameprop.default = 'hello';
  servicenameprop.type = 'String';
  
  const serviceNameParameter = new Parameter(this, 'servicename', servicenameprop);
  serviceNameParameter.value = new Token(servicename, 'servicename');
  </code></pre>
  
  <p>This results in parameters appearing in the CloudFormation dashboard tab, but there are no values set, just the defaults. Is this supported currently?  If not, is it planned for the future?</p>