redis::instance: do not exec cp if source and destination are equal

[trefzer]

Pull Request (PR) description

Avoid using this copy with destination and source beeing equal

Remark: generally it's a bad idea to use an exec statement to copy a file. Since this way we cannot see diffs etc. A better way would be to use a file{ $destination: source=> $source} statement, but running with noop makes it fail then !