Cleanup hack/test-integration.sh

Proper quoting, error checks, use `$KUBE_*` variables when applicable, trap to do
cleanup on all errors, call cleanup explicitly on successful exit.

Tested:
- Ran it manually to completion, test run successful.
- Interrupted it during the run, checked that `$?` was non-zero, etcd was killed.

@jbeda @eparis @smarterclayton who worked on this script before.