Mark Elastic Transcoder required params

This PR supersedes #244, fixes #214 and marks all the required Elastic Transcoder params as such via its `.extra.json` file. After the change the CLI looks like:

```bash
$ ./bin/aws elastictranscoder delete-pipeline
usage: aws [options] <command> <subcommand> [parameters]
aws: error: argument --id is required
```

Raw botocore looks like:

```python
>>> import botocore.session
>>> s = botocore.session.get_session()
>>> e = s.get_service('elastictranscoder')
>>> op = e.get_operation('DeletePipeline')
>>> endpoint = e.get_endpoint('us-east-1')
>>> op.call(endpoint)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "botocore/operation.py", line 62, in call
    params = self.build_parameters(**kwargs)
  File "botocore/operation.py", line 172, in build_parameters
    object_name=self)
botocore.exceptions.MissingParametersError: The following required parameters are missing for Operation:DeletePipeline: id
```

This is my first edit of the `.extra.json` files so let me know if anything else needs to be done @jamesls.

It looks like Elastic Transcoder is the only service marked as `rest-json`.