Use ginkgo's native support for JUnit in order to generate the XML file.

This is a first step in better integration of our e2e tests with
Jenkins. In order to improve the logged information, we will probably
need to have more native ginkgo tests but this step allows us to see
what Jenkins can already do with this information and what we need to
tweak to improve it.

Tested by running the full e2e tests and inspecting the contents of
junit.xml on the top of the tree.

Textual output is still generated on the console to keep the current
goe2e.sh logs available until the full conversion of our Jenkins
instance to use the JUnit XML is completed.