Allow the post_fail_hook ip command to fail

[perlpunk]

It may fail in case this IP is not actually set on the bridge.
Also the ctrl-c needs to happen before, because if the

grep ... tail -f -n0 nohup.out

fails, it's not killed. Since ctrl-c shouldn't do any harm, I guess we can
do it regardless of which test we are running.

This ensures that the post_fail_hook can be completed.

Also this fixes sporadic failures in openqa_from_git: https://openqa.opensuse.org/tests/3946911#step/openqa_webui/36

[perlpunk]

Ok, now the post_fail_hook suceeds.
https://openqa.opensuse.org/tests/3947127#step/openqa_webui/49
Of course that still doesn't solve the problem, so it needs to be merged manually.
However, since I added changes (moved the ctrl-c), I will wait a bit.

[perlpunk]

Added another commit that improves the grepping of nohup.out, this fixes these sporadic failures:
https://openqa.opensuse.org/tests/3946911#step/openqa_webui/36

The content of the nohup.out is:

[info] Listening at "http://127.0.0.1:9526"
Web application available at http://127.0.0.1:9526
[info] Listening at "http://[::1]:9526"

So everything worked, but it still ran into the timeout, that's why I realized that the while / grep logic is broken.

[okurz]

this fails with the error that is fixed in #166 and #166 fails with the error that is fixed here :D Let's merge both and see how the current state in master will look like after merge.