Switch job configuration section to use a checkbox

This pull request changes the plugins job configuration from using a _section_ to an _optionalBlock_. In effect, it makes the plugins configuration fall inline with the convention used by most other plugins.
Thanks for the PR!
I think it is possible to remove the need for adding a new member for this class than also needs to be serialized to the configuration file of the project. It could break the Jenkins Job builder and Job DSL plugin project generation.
Is it possible to remove the configEnabled member instead have this in the method?
```
        return taskName != null && !taskName.equals("") || stageName != null && !stageName.equals("");
```