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.

Signed-off-by: Filipe Brandenburger <filbranden@google.com>
Merge pull request #1412 from filbranden/test_integration1

Cleanup hack/test-integration.sh