This commit silences logrus logging for now by just dicarding
everything from it. The rational is that the underlying libraries
produce a lot of output, e.g. every build currently results in
```console
$ image-builder build qcow2
WARN[0000] Failed to load consumer certs: no consumer key found
...
```
which is confusing to our users. What is worse is that containers
also uses logrus so we can get random warnings without context
from there too. Until we have a way to filter log messages this
seems to be the most practical way.
We can add `--verbose` or `--debug` later to enable these kinds
of messages.
c590c38d4f