[FIXED JENKINS-17875] added variables jenkins, project in SystemGroovyChoiceListProvider.
[JENKINS-17875] Improved the way to retrieve (and keep) project.
[JENKINS-17875] Added option whether to use predefined variables. This can allow users to have a workaround  when a problem happened retrieving project.
Merge pull request #5 from ikedam/feature/JENKINS-17875_VariablesInGroovy

[JENKINS-17875] Referencing project from System Groovy Script.