Fixes for `e2e-tests.sh`: (#1303)
Fixes for `e2e-tests.sh`: (#1303)

* Don't require SSH keys for running `e2e-tests.sh`. GCP SSH keys are not used, but `kubetest` checks for their existence. When running the files locally, create empty files (if necessary) to satisfy the check.
* Rename `exit_if_failed` to `abort_if_failed`
* Don't dump stack info when exiting the script when a test failed, it's redundant and `run_e2e_test` already does that
knative/serving,knative/serving