Check if cluster is up before e2e test without -up

Minor usability nuisance: If you run:
  go run hack/e2e.go -v -test
.. and you don't happen to have an up e2e cluster, it should fail
fast, rather than chugging through every test and having them fall
over.
Merge pull request #2783 from zmerlynn/IsUp

Check if cluster is up before e2e test without -up