Code review comment for ~pwlars/checkbox/+git/support:test-in-lxd

New improvements!
- More consistent output, container names on all lines of output
- No more random sleeps, now we just wait for network activation in the containers
- Return of kissiel's parallel execution patch, which makes a much bigger difference now that io overhead is reduced

Total overall testing time on checkbox-ng is now down to about 3m30s