Parameterize jenkins plugin repository and add support for core plugins

[backuitist]

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. When necessary plugins can use a different groupId:

dynamicparameter : {
    version: "0.2.0",
    group_id: "com.seitenbau.jenkins.plugins"
}

As for the core plugin support this should fix #118
Simply set core to the core plugins that you want to upgrade:

matrix-auth : { 
    version: "1.2",
    core: true
}

[jenkinsadmin]

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[jhoblitt]

This functionality was merged in #190. This PR can be closed.

[jhoblitt]

@backuitist Actually, #190 didn't fully cover this functionality. However, I'm going to close this PR due to code drift and lack of activity. Please feel free to reopen it in the future.