LaunchConfig's Metadata missing Comment field

Hello! I currently have a CF template that has comments inside the metadata of a launch configuration. I'm unable to produce the same template via troposphere:

```
LaunchConfig = t.add_resource(LaunchConfiguration(
"Foo",
Metadata=Metadata(Comment="Testing")
))
```

Yields `__init__() got an unexpected keyword argument 'Comment'`.

I'm looking for this to be produced (which seems valid on AWS itself):

```
"LaunchConfig": {
"Metadata": {
"Comment": "Foo"
...
```

Start from the LaunchConfiguration and Metadata entry points in troposphere and compare the accepted Metadata fields with the requested Comment field. Reproduce the shown template output and verify that a LaunchConfiguration can emit Comment under Metadata.

Written by the indexing model from the issue text.