pep8: exclude .git/ subdirectory
pep8: exclude .git/ subdirectory

When a git branch is called trove-xxx, "tox -e pep8" fails on
checking .git/logs/refs/heads/trove-xxx. It doesn't make sense to
validate Git branches using flake8.

Simply exclude .git from flake8.

Note: [testenv:pep8] of tox.ini looks for "trove-*" to validate
contrib/trove-guestagent.

Change-Id: I7c5582282a63da8cc1b3bbb6b76a267ef894a5d7

mmasaki/trove,openstack/trove,mmasaki/trove,openstack/trove,zhangg/trove,hplustree/trove,hplustree/trove,mmasaki/trove,zhangg/trove