Use the E2E_REPORT_DIR global environment variable to define the
location where the JUnit XML reports should be saved.
Modify the Jenkins e2e.sh script to export that variable pointing to the
top of the Jenkins build tree.
Tested by running `E2E_REPORT_DIR=${PWD}/.. hack/e2e-test.sh` and
confirmed ../junit.xml was generated and looked good.
And a config description. This doesn't yet have much - first want to
make sure I can do the build job. Next I'll submit the e2e script with
its twiddles and switch those over. (After going to 3 e2es, I think
it's finally time for version control.)
