Allow keeping sct-runner after the test

[soyacz]

When test ends and logs are collected, sct-runner will be terminated immediately.
We may want to keep it and we have no means to prevent from marking runner with keep: 0 which happens in pipeline clean sct-runner step.
Idea is to add a way to allow keeping runner (might be useful in docker based tests, or kafka ones) and document it.

refs: #8912

[fruch]

We should be able to tag manually with keep: alive, and test shouldn't remove/replace it during cleanup/teardown