Horizon and most horizon plugins support Django 2.0 in Rocky.
This commit focuses on passing the unit tests with Django 2.0.
It would be nice if masakari-dashboard team tests it in
real environments.
Replace django.core.urlresolves with django.urls
(In Django 2.0) The django.core.urlresolvers module is removed
in favor of its new location, django.urls.
It was deprecated in Django 1.10:
https://docs.djangoproject.com/en/2.0/releases/1.10/#id3
Drops older Django unit tests from tox.ini
as horizon dropped Django <=1.10 support in Rocky.
Change-Id: I67907a2460f8472e5a6222210d74b3eed71be064
4052bde700