[CustomScript] extension should allow for a script payload

Given that uploading dependencies to an added storage account may be unfeasible (or just too much overhead) when using CLI scripting, this extension should support the use case of inlining a shell script as part of an ARM template - something like the example below would go a very long way towards matching [this competing feature](http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/user-data.html#user-data-shell-scripts) and allowing for easy migration.

``` json
"settings": {
"inlineScript": "#!/bin/bash\nsudo apt-get -y update\nsudo apt-get install nginx"
}
```

No implementation files or tests are named in the issue. Start by locating the CustomScript extension's settings handling and trace how ARM template settings launch scripts; done means an inlineScript value can be supplied in the shown settings block and executed without uploading a separate dependency payload.

Written by the indexing model from the issue text.