Support for Managed Maven Settings

This add initial support for the [Config File Provider Plugin](https://wiki.jenkins-ci.org/display/JENKINS/Config+File+Provider+Plugin):

```groovy
job {
    steps {
        maven {
            providedSettings 'company-settings'
        }
    }
}

job(type: Maven) {
    providedSettings 'company-settings'
}
```