refactor(build): replace source in environments with environmentDefault entry

Replace source in environments with separate environmentDefault entry and updated docs to reflect this change

BREAKING CHANGE: angular-cli.json has changed. A new environmentDefault entry replaces the previous source entry inside environments.

To migrate the code follow the example below:

Before:

"environments": {
"source": "environments/environment.ts",
	"dev": "environments/environment.ts",
	"prod": "environments/environment.prod.ts"
}

After:

"environmentSource": "environments/environment.ts",
"environments": {
	"dev": "environments/environment.ts",
	"prod": "environments/environment.prod.ts"
}

Closes angular#3857

Author: jsanchezgarcia

packages/angular-cli/models/webpack-build-common.ts

@@ -103,7 +103,8 @@ export function getWebpackCommonConfig(
   // process environment file replacement
   if (appConfig.environments) {
     if (!appConfig.environmentSource) {
-      throw new SilentError(`Environment configuration does not contain "environmentSource" entry.`);
+      throw new SilentError(`Environment configuration 
+        does not contain "environmentSource" entry.`);
     }
     if (!(environment in appConfig.environments)) {
       throw new SilentError(`Environment "${environment}" does not exist.`);