Parameterize jenkins plugin repository and add support for core plugins

This is useful when you want jenkins to download plugins from an enterprise repository.
Repository base url can be redefined and accept a maven style layout. Plugins can then use a different groupId:

    dynamicparameter : {
        version: "0.2.0",
        group_id: "com.seitenbau.jenkins.plugins"
    }

As for the core plugin support this should fix
https://github.com/jenkinsci/puppet-jenkins/issues/118
Simply set core to the core plugins that you want to upgrade:

    matrix-auth : { 
        version: "1.2",
        core: true
    }