Add tox fast8 option

This commit ports over the fast8 option from cinder, which enables
one to run pep8 only on one's changeset rather than against the
whole body of code via:
     tox -efast8

Change-Id: Icb0286c3c66983f2f106c163cb11c690b2c57849

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

@@ -55,6 +55,10 @@ commands = python setup.py build_sphinx
 [testenv:cover]
 commands = {toxinidir}/tools/cover.sh {posargs}
 
+[testenv:fast8]
+commands =
+  {toxinidir}/tools/fast8.sh
+
 [testenv:pylint]
 deps = -r{toxinidir}/requirements.txt
        pylint==0.26.0