feat(cli): Allow specifying options using env vars (#1447)

Allows passing options to the `cdk` command using environment variables
with the `CDK_` name prefix. This can be used, for example, for the
following:

- `CDK_VERSION_REPORTING=false` disables version reporting
- `CDK_REQUIRE_APPROVAL=broadening` requires approval for changes to
  security policies that broaden the grants
- `CDK_VERBOSE=true` enables verbose logging in the toolkit

Added a validator in `cdk doctor` that will display the environment
variables that may be picked up by the toolkit, and highlight those that
are reserved for internal use by the toolkit (`CDK_CONTEXT_JSON` and
`CDK_OUTDIR`).

packages/aws-cdk/bin/cdk.ts

@@ -30,6 +30,7 @@ const DEFAULT_TOOLKIT_STACK_NAME = 'CDKToolkit';
 async function parseCommandLineArguments() {
   const initTemplateLanuages = await availableInitLanguages;
   return yargs
+    .env('CDK')
     .usage('Usage: cdk -a <cdk-app> COMMAND')
     .option('app', { type: 'string', alias: 'a', desc: 'REQUIRED: Command-line for executing your CDK app (e.g. "node bin/my-app.js")' })
     .option('context', { type: 'array', alias: 'c', desc: 'Add contextual string parameter.', nargs: 1, requiresArg: 'KEY=VALUE' })

packages/aws-cdk/lib/commands/doctor.ts

@@ -1,3 +1,4 @@
+import cxapi = require('@aws-cdk/cx-api');
 import colors = require('colors/safe');
 import process = require('process');
 import yargs = require('yargs');
@@ -25,7 +26,8 @@ export async function realHandler(_options: CommandOptions): Promise<number> {
 
 const verifications: Array<() => boolean | Promise<boolean>> = [
   displayVersionInformation,
-  displayAwsEnvironmentVariables
+  displayAwsEnvironmentVariables,
+  displayCdkEnvironmentVariables,
 ];
 
 // ### Verifications ###
@@ -47,3 +49,22 @@ function displayAwsEnvironmentVariables() {
   }
   return true;
 }
+
+function displayCdkEnvironmentVariables() {
+  const keys = Object.keys(process.env).filter(s => s.startsWith('CDK_'));
+  if (keys.length === 0) {
+    print('ℹ️ No CDK environment variables');
+    return true;
+  }
+  print('ℹ️ CDK environment variables:');
+  let healthy = true;
+  for (const key of keys.sort()) {
+    if (key === cxapi.CONTEXT_ENV || key === cxapi.OUTDIR_ENV) {
+      print(`  - ${colors.red(key)} = ${colors.green(process.env[key]!)} (⚠️ reserved for use by the CDK toolkit)`);
+      healthy = false;
+    } else {
+      print(`  - ${colors.blue(key)} = ${colors.green(process.env[key]!)}`);
+    }
+  }
+  return healthy;
+}