Improve debuggability of integration test failures

[zcahana]

Current integration test output is very limited (simple echo's and set -x), which makes it very hard to debug test failures on Travis. We can improve this by:

1. Dumping information on what containers/pods are running.
2. Dumping stdout/stderr of containers.
3. Dumping relevant log files (nginx's, envoy's, ...).
4. Remove the clutter output from set -x, and replace it with more focused prints.