avocado: Add NullHandler for early 'avocado.test' logger

So we avoid those annoying 'no handlers found for avocado.test'
during test.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #191 from lmr/fix-logging-handlers

avocado: Add NullHandler for early 'avocado.test' logger