fix(tox): move dependency to requirements.txt

fix(tox): move dependency to requirements.txt

Two reasons.
Most importantly, executor image creation scripts
will now see the pylint version used in tox job.
Additionally, this makes it easier for people
to run pylint locally (without checking all files as tox does).

Change-Id: I89f1d428e676e27d6f35c79ca9c22224c4c99a3e
Signed-off-by: Vratko Polak <vrpolak@cisco.com>