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 and
lower-constraints.txt are updated accordingly. for more info. please
refer [1].
Depends-On: https://review.opendev.org/#/c/700733/
[1] https://etherpad.openstack.org/p/Enable_Django22_support_in_Horizon_Plugin

Change-Id: I0369b8726b579ae46a439c83d953ec03d2f981ff

lower-constraints.txt

@@ -2,7 +2,7 @@ Babel==1.3 # BSD
 coverage==3.6 # Apache-2.0
 ddt==1.0.1 # MIT
 hacking==1.1.0 # Apache-2.0
-horizon==14.0.0.0b2 # Apache-2.0
+horizon==17.1.0 # Apache-2.0
 nodeenv==1.3.2  # BSD
 openstackdocstheme==1.18.1 # Apache-2.0
 oslo.log==3.36.0 # Apache-2.0

requirements.txt

@@ -8,4 +8,4 @@ Babel>=1.3 # BSD
 
 # This will be installed from git in OpenStack CI if the job setting
 # required-projects for horizon:
-horizon>=14.0.0.0b3 # Apache-2.0
+horizon>=17.1.0 # Apache-2.0

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.0
-envlist = py37,py3-{dj111,dj22},pep8,docs
+envlist = py37,pep8,docs
 skipsdist = True
 
 [testenv]
@@ -15,8 +15,6 @@ deps =
   -r{toxinidir}/requirements.txt
   -r{toxinidir}/test-requirements.txt
 commands =
-  dj111: pip install django>=1.11,<2
-  dj22: pip install django>=2.2,<2.3
   python manage.py test
 
 [testenv:pep8]