Drop Django 1.11 support

Django 1.11 ends its extended support in April 2020 (which is before
Ussuri release), so horizon drops Django 1.11 support in Ussuri.

tox envs for non-primary Django versions are no longer needed in tox.ini
as testing environments for non-primary Django versions are setup in
the zuul jobs now.

horizon>=17.1.0 is required to use Django 2.2.
requirements.txt is updated accordingly.

Depends-On: https://review.opendev.org/#/c/700733/
Change-Id: I8f97ac0e749c6c108d809f246b044ba3e8a0010d

requirements.txt

@@ -6,3 +6,4 @@ pbr!=2.1.0,>=2.0.0 # Apache-2.0
 iso8601>=0.1.11 # MIT
 python-mistralclient!=3.2.0,>=3.1.0 # Apache-2.0
 PyYAML>=3.12 # MIT
+horizon>=17.1.0  # Apache-2.0

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.0
-envlist = pep8,py37,py3-{dj111,dj22}
+envlist = pep8,py37
 skipsdist = True
 
 [testenv]
@@ -13,8 +13,6 @@ deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
        http://tarballs.openstack.org/horizon/horizon-master.tar.gz
 commands =
-    dj111: pip install django>=1.11,<2
-    dj22: pip install django>=2.2,<2.3
     find . -type f -name "*.pyc" -delete
     /bin/bash run_tests.sh -N --no-pep8 {posargs}
 whitelist_externals = find