Allow overriding releaseVersion for release-finish

[FichteFoll]

The release and release-start goals have a releaseVersion parameter that can be used to set and modify the version used in the process, but the release-finish (and neither hotfix-finish) do. I would like to reserve the right to overwrite whatever the pom has with the release branch name as editing the pom is frequently forgotten whereas using the correct branch name is not (if the release branch was created manually).