Tox: Add GPL license checker

A common mistage is to add a file into GPL directory,
with Apache license only.
This adds a tox checker to vote -1 on such changes,
gpl_license.log contains the list of affected files.

There are TODOs related to unlicensed files
or unconventional GPL licenses.

Change-Id: I267e4fb98afd199d5acc7d211068b28c97b31dc9
Signed-off-by: Vratko Polak <vrpolak@cisco.com>