Add ability to run pylint check on modified files only

Add ability to run pylint check on modified files only

Change c0aa0b fused pylint with the checks done with the pep8 testenv.
This has the drawback of increasing the execution time of the command
'tox -epep8' fourfold.

This change introduces the ability to execute the pylint check on just
the list of python files affected by a specific patch. For instance,
this can be done with the following command:

   tox -epep8 HEAD~1

A kind reminder is added, so that folks who end up noticing the spike of
time execution when they run tox -epep8 without posargs, can learn how
they can get their precious minutes back.

Change-Id: Ie2a2b3a0171908d8d8aa8713baaa875adccb7efe