Enhanced rudimentary support for custom config files

Enhanced support of [Config File Provider Plugin](https://wiki.jenkins-
ci.org/display/JENKINS/Config+File+Provider+Plugin) to enable custom
config files for build jobs.

```groovy
job {
    wrapper {
        configFile {
            fileName 'myCustomConfigFile'
        }
    }
}

```
cleanup, allow to add multiple files
Merge branch 'config-file'