support TemplateSpecification element

* this is what EB CLI uses to create an RDS resource as part of CreateEnvironment
* exposes a --template-specification option on aws elasticbeanstalk create-environment

Example usage:

    aws elasticbeanstalk create-environment ... --template-specification file://rds.us-west-2.json

where rds.us-west-2.json is:

    {
      "TemplateSnippets": [{
        "SnippetName": "RdsExtensionEB",
        "Order": 10000,
        "SourceUrl":
    "https://s3.amazonaws.com/elasticbeanstalk-env-resources-us-west-2/eb_snippets/rds/rds.json"
        }]
    }