Eliminate unnecessary deps for tox -e pep8,genconfig

Purpose: dramatically reduce how long it takes to run
$ tox -e pep8,genconfig
improving usability and dev productivity.

Change:
Replace full requirements.txt + test-requirements.txt install by minimal deps
for pep8 and genconfig testenv's

Result:
- tox -epep8 time reduces from 3m30s to 16s on initial run and
from 50s to 5s on subsequent runs
- tox -egenconfig time reduces from 3m46s to 12s on initial run and
from 1m13s to 3s in subsequent runs
(Times measured on my machine; similar proportions expected on other machines)

Change-Id: I0596a6970725189fd9b46a0cac923b86ccb03f8c

tools/pip-install-single-req.sh

@@ -0,0 +1,5 @@
+#!/bin/sh
+
+# install specific package $2 according to
+# version specified in requirements file $1
+pip install -U `grep $2 $1 | sed 's/#.*//'`

tox.ini

@@ -20,7 +20,10 @@ commands =
  python setup.py testr --slowest --testr-args='{posargs}'
 
 [testenv:pep8]
-commands = flake8
+usedevelop = False
+deps =
+commands = {toxinidir}/tools/pip-install-single-req.sh test-requirements.txt hacking
+           flake8
 
 [testenv:venv]
 commands = {posargs}
@@ -44,4 +47,7 @@ builtins = _
 exclude=.venv,.git,.tox,dist,doc,*openstack/common*,*lib/python*,*egg,build,*thirdparty/*,CongressLexer.py,CongressParser.py,contrib/*
 
 [testenv:genconfig]
-commands = oslo-config-generator --config-file=etc/congress-config-generator.conf
+usedevelop = False
+deps =
+commands = {toxinidir}/tools/pip-install-single-req.sh requirements.txt oslo.config
+           oslo-config-generator --config-file=etc/congress-config-generator.conf