Fix and reorganize tox env definitions
Fix and reorganize tox env definitions

* Define py35 commands in a single place.
  Previously it was defined in [testenv] and [testenv:py35].
* py35 env should use python3.5 instead of python3
  because pyNN job is specific to a specific python version.
  In addition, python version is automatically assumed by tox.
* Sort env definitions in the order of unit tests, other testing stuffs
  with 'testenv:' prefix and finally non testenv stuffs (like flake8).
* Drop testenv:manage-py35dj20 because testenv:manage now uses
  python 3.5 and Django 2.0.
* Avoid using dict-style env commands definitions.
  In OpenStack projects, we usually use [testenv:xxxx] style.

Change-Id: Ib55031a50a51f952b738f0fdf33313fa2c7522ff

ChameleonCloud/horizon,NeCTAR-RC/horizon,noironetworks/horizon,noironetworks/horizon,noironetworks/horizon,ChameleonCloud/horizon,noironetworks/horizon,openstack/horizon,openstack/horizon,ChameleonCloud/horizon,openstack/horizon,NeCTAR-RC/horizon,NeCTAR-RC/horizon,ChameleonCloud/horizon,openstack/horizon,NeCTAR-RC/horizon