[MRELEASE-1123] Use the PlexusConfiguration interface instead of the final class

[gnodet]

https://issues.apache.org/jira/browse/MRELEASE-1123

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[hboutemy]

let's merge this update, even if it fixes only one issue on the multiple ones from MRELEASE-1123
small PRs are easier to review

[slawekjaranowski]

suppressed by #186