[FIXED JENKINS-19926] Add only new env. variables

Previous patch (3eea39648943583a530f47f69f93737266537d9d) broke usescases when evn. variable overwrites already existing one
like PATH = $PATH:/my/path
[maven-release-plugin] prepare release jenkins-1.535
[maven-release-plugin] prepare for next development iteration
updated changelog for release
updated changelog as a part of the release
merged back the RC branch
creating an RC branch
the trunk is toward 1.537-SNAPSHOT
Merge pull request #970 from vjuranek/JENKINS-19926

[FIXED JENKINS-19926] Add only new env. variables
[FIXED JENKINS-20071] Upgrade bundled versions of credentials and ssh-slaves so we can assume <c:select/> available
[JENKINS-18629] Noting retroactively.