avocado: Fix mistakes in copyright headers

I went through all python files in avocado code, and
some changes were made:

* Files entirely written by the current avocado dev
team were changed to be GPL v2+
* Files with a significant portion of code derived
from autotest remain GPL v2 strict, and original
authorship was re-established

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>