tools/pre-commit: Add pep8 to check python files

Pep8 checks for different set of errors then pylint, and is small
and fast, so it seems to be worthy addition.

This commit also abandons ad-hock checks for trailing whitespace,
as pep8 does exactly this for python files, and we don't have
any html or js files here.

Change-Id: I862b26f411f26a3a279c4f7835e7892a45e28994
Merge "tools/pre-commit: Add pep8 to check python files" into stable/grizzly