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: Ide8df87a29eb94eb17ea9d132a1162f2915948ee

lower-constraints.txt

@@ -14,7 +14,7 @@ ddt==1.0.1
 debtcollector==1.19.0
 decorator==4.2.1
 deprecation==2.0
-Django==1.11
+Django==2.2
 django-appconf==1.0.2
 django-babel==0.6.2
 django-compressor==2.0
@@ -28,7 +28,7 @@ fixtures==3.0.0
 flake8==2.5.5
 futurist==1.6.0
 hacking==0.12.0
-horizon==14.0.0.0b3
+horizon==17.1.0
 idna==2.6
 imagesize==1.0.0
 iso8601==0.1.11

requirements.txt

@@ -12,4 +12,4 @@ python-keystoneclient>=3.8.0 # Apache-2.0
 python-manilaclient>=1.16.0 # Apache-2.0
 six>=1.10.0 # MIT
 
-horizon>=14.0.0.0b3 # Apache-2.0
+horizon>=17.1.0  # Apache-2.0

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 1.6
-envlist = py37,pep8,py3-{dj111,dj22}
+envlist = py37,pep8
 skipsdist = True
 
 [testenv]
@@ -13,8 +13,6 @@ deps = -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/
        -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
   /bin/bash run_tests.sh -N --no-pep8 {posargs}
 
 [testenv:py36]