Add fast8 to quickly test pep8 changes

This patch adds the 'fast8' test that Cinder has to os-brick.
It's a simple pep8 check against what has changed since HEAD.
This allows you to test for pep8 violations in files that you have
changed without testing the entire repo.

Change-Id: I4e3840d763d08a80c84ba00e2d5e5710a905ecf3

tools/fast8.sh

@@ -0,0 +1,15 @@
+#!/bin/bash
+
+cd $(dirname "$0")/..
+CHANGED=$(git diff --name-only HEAD~1 | tr '\n' ' ')
+
+# Skip files that don't exist
+# (have been git rm'd)
+CHECK=""
+for FILE in $CHANGED; do
+    if [ -f "$FILE" ]; then
+        CHECK="$CHECK $FILE"
+    fi
+done
+
+diff -u --from-file /dev/null $CHECK | flake8 --diff

tox.ini

@@ -22,7 +22,12 @@ whitelist_externals = bash
 passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
 
 [testenv:pep8]
-commands = flake8
+commands = flake8 {posargs}
+
+[testenv:fast8]
+envdir = {toxworkdir}/pep8
+commands =
+    {toxinidir}/tools/fast8.sh
 
 [testenv:pylint]
 deps = -r{toxinidir}/requirements.txt