Add "fast8" tox env

This replicates the run_tests.sh -8 behavior, running
flake8 only on changes made in the last commit and
working tree.

Change-Id: Iebd5746c78e840225f0860843c77f4c638877c26

tools/fast8.sh

@@ -0,0 +1,5 @@
+#!/bin/bash
+
+cd $(dirname "$0")/..
+CHANGED=$(git diff --name-only HEAD~2 | tr '\n' ' ')
+diff -u --from-file /dev/null $CHANGED | flake8 --diff

tox.ini

@@ -114,6 +114,10 @@ commands =
   {toxinidir}/tools/config/check_uptodate.sh
   {toxinidir}/tools/check_exec.py {toxinidir}/cinder
 
+[testenv:fast8]
+commands =
+  {toxinidir}/tools/fast8.sh
+
 [testenv:pylint]
 deps = -r{toxinidir}/requirements.txt
        pylint==0.26.0