Fix for https://issues.jenkins-ci.org/browse/JENKINS-8535 - do not fail grails builds with failing tests; fix for https://issues.jenkins-ci.org/browse/JENKINS-10266 - Set grails.work.dir to something unique by default

Does what it says on the tin:

1. allows JUnit reporter to mark builds which build but have failing tests as UNSTABLE vs STABLE, rather than them all being FAILED.

2. if no grails.work.dir is specified either thru jenkins settings or BuildConfig, will be set to $WORKSPACE/target.