Update tox.ini to current standards

Remove pyflakes env - it's covered by flake8. Use custom install_command
to avoid unwanted pre-release software. Skip the unneeded sdist step.

Change-Id: I2c86dabeb78da9f1a9ee7a8636bc5d58b2439d20

tox.ini

@@ -1,12 +1,14 @@
 [tox]
 minversion = 1.6
-envlist = pep8, pyflakes, py27
+skipsdist = True
+envlist = pep8, py27
 
 [testenv]
 # Set STATSD env variables so that statsd code paths are tested.
 setenv = STATSD_HOST=localhost
          STATSD_PORT=8125
          VIRTUAL_ENV={envdir}
+usedevelop = True
 install_command = pip install {opts} {packages}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
@@ -23,11 +25,6 @@ commands = flake8
 commands =
   python setup.py testr --coverage
 
-[testenv:pyflakes]
-deps = pyflakes
-       -r{toxinidir}/requirements.txt
-commands = pyflakes zuul setup.py
-
 [testenv:venv]
 commands = {posargs}