feat(aws-certificatemanager): add DNSValidatedCertificate (#1797)

Add a new class to `@aws-cdk/aws-certificatemanager` called
`DNSValidatedCertificate`.  This class generates a certificate request
using AWS Certificate Manager and auto-validates the request using the
provided DNS "magic cookie" records.  The user need only supply the
Domain Name of the certificate and a Route 53 Hosted Zone.  A
CloudFormation Custom Resource is used along with the supplied Lambda
function to perform the operation.

Fixes #605.

lerna.json

@@ -3,6 +3,7 @@
  "packages": [
  "packages/*",
  "packages/@aws-cdk/*",
+ "packages/@aws-cdk/*/lambda-packages/*",
  "tools/*"
  ],
  "rejectCycles": "true",