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: I499c5f2fb06caa70c939b6e503f65c14c2c2e474

lower-constraints.txt

@@ -8,7 +8,7 @@ cmd2==0.8.1
 colorama==0.3.9
 coverage==4.0
 debtcollector==1.19.0
-Django==1.11
+Django==2.2
 django-appconf==1.0.2
 django-compressor==2.0
 docutils==0.14
@@ -16,6 +16,7 @@ extras==1.0.0
 fixtures==3.0.0
 flake8==2.5.5
 hacking==0.12.0
+horizon==17.1.0
 idna==2.6
 imagesize==1.0.0
 iso8601==0.1.12

requirements.txt

@@ -6,4 +6,4 @@ oslo.log>=3.36.0  # Apache-2.0
 
 # OpenStack CI will install the following projects from git
 # if they are in the required-projects list for a job:
-horizon>=14.0.0.0b3 # Apache-2.0
+horizon>=17.1.0 # Apache-2.0

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py36,py37,py3-{dj111,dj22},pep8,pylint,docs
+envlist = py36,py37,pep8,pylint,docs
 minversion = 2.0
 skipsdist = True
 
@@ -16,8 +16,6 @@ deps = -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/re
        -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 {posargs}
 
 [testenv:py36]