minor: update checkstyle to 8.0 and intoroduce checkstyle.version variable (#1686)
minor: update checkstyle to 8.0 and intoroduce checkstyle.version variable (#1686)

first steps to use latest version of checkstyle , base on conversation at https://groups.google.com/forum/#!topic/checkstyle/KYF-9EEzsbs
jmptrader/Strata,OpenGamma/Strata