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

This is an implementation for [JENKINS-17875](https://issues.jenkins-ci.org/browse/JENKINS-17875).
You can now access the information of the project in groovy scripts.
As this is done in a tricky way, this feature is disabled by default , and you need to check "Use variables in groovy scripts" in a project configuration page to enable it.