Code Cleanup: Enable test parallelization by removing shared test state

This is similar to https://github.com/jenkinsci/google-kubernetes-engine-plugin/issues/27

Here the main focus will be on [ComputeEngineCloudIT.java](https://github.com/jenkinsci/google-compute-engine-plugin/blob/master/src/test/java/com/google/jenkins/plugins/computeengine/ComputeEngineCloudIT.java) because they take the most time. I'll also look at addressing #40 so that I don't have to do this change twice.