Drop Django 1.4 support in L

Summary:
Drop Django 1.4 support in L

Motivation:

Django is dropping support for Django 1.4 in October 2015 prior to release of L. We
should just drop support of Django1.4 earlier in the requirements.txt and remove the Django 1.4
gate job.

https://www.djangoproject.com/weblog/2015/apr/01/release-18-final/

"With the release of Django 1.8, Django 1.6 has reached end-of-life. As such, Django 1.6.11 is the final release of the 1.6 series. Django 1.7 will continue to receive security updates until the release of Django 1.9 (planned for October 2015). Django 1.4 (the previous LTS) will receive security updates for another six months (ending October 1, 2015) to give time for users to upgrade to Django 1.8 LTS."

Description:

There are a number of items to be implemented:
1. Fix horizon and django_openstack_auth to work with Django 1.8
2. Drop Django 1.4 in requirements and the gate job
3. Both Django 1.5 and Django 1.6 reach end-of-life, change the minimum version to Django 1.7
4. Change the gate job to run django17

UX:
N/A

Outside Dependencies:
Django18 LTS release

Requirements Update Required:
N/A

Doc Impact:
Update docs about the bump in minimum Django version supported.