Make pass scenario more readable

There is too much bloat info in the passing scenario, much of it is duplicit or very useless when scenario passed.

Also, fail_only argument was happily ignored till now, after fixing that and adding NO_EMBED to some process calls (when result is embedded anyway or has no real use), and grouping testeth0 commands from environment.py:before_scenario into testeth0 check, the output looks much cleaner:

With NMCI_DEBUG=y in env (should equal to failing scenario), the output is much more verbose (more journal logs, statuses, file contents etc.):