E2E test: easy way to bring the testenv up and down

[mflendrich]

per #869 (review)

Bringing an environment online from scratch isn't particularly quick (around 3 minutes before it starts actually running tests for me). That's probably unavoidable, but keeping an existing environment online has its own challenges, as there's no single step to run one test only: you must set up environment variables, port-forwards, and run scripts all by hand. There's furthermore no built-in means to tear an environment down (running ./kind delete cluster --name test-cluster; docker rm -f $(docker ps -a -q); docker rmi $(docker images -a -q) appears good enough).

May be as simple as documenting usage of scripts under /test/integration/util.

[mflendrich]

Obsoleted by Kong/keps#11