Django 2.0 support

Summary
=======
The goal of this blueprint is to make horizon work with Django 2.0.
We will support Django 1.11 (LTS) and Django 2.0+.

Motivation
========

Debian will switch the default Django version to 2.x soon. Although Django 2.0 is not LTS, it seems worth considering support of Django 2.0.

Apart from 1.11, Django 2.0 is the only release supported by Django community.
Django changes a lot release by release, so it sounds better to catch up with the changes as much as possible.

Description
=========

The following works are planned:

* Catch up with Django 2.0 features and handle removed interface in Django 2.0.
* More python 3 support. Django 2.0 requires python 3, and if we test more with Django 2.0 some modification for python 3 might be needed.
* Drop Django <= 1.10 support because Django 2.0+ and Django <=1.10 are not compatible. We will drop Django <=1.10 support in Queens, so it is not a problem. Precisely this is not a part of this blueprint.

UX
===
No UX impact.

Testing
======
Existing unit test covers the change. We will add py35dj20 tox env.

In addition, tox manage-py35dj20 env will be added so that we can use runserver with Django 2.0 easily.

Outside Dependencies
==================
Django 2.0 which was release this December.
The default version is kept to Django 1.11 (which is LTS).

Requirements Update Required
========================
The default version is kept to Django 1.11 so there is no g-r change required.

Doc Impact
=========
Needs to update the dependencies section in the installation guide (which is already obsolete now :/
https://docs.openstack.org/horizon/latest/install/from-source.html#system-requirements