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: I55754649f39234f65b310c5a7f13efbc090bd9cd

requirements.txt

@@ -12,4 +12,4 @@ python-qinlingclient>=1.1.0 # Apache-2.0
 
 # This will be installed from git in OpenStack CI if the job setting
 # required-projects for horizon:
-horizon>=14.0.0.0b1 # Apache-2.0
+horizon>=17.1.0 # Apache-2.0

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py37,py3-{dj111,dj22},pep8,docs
+envlist = py37,pep8,docs
 minversion = 2.3.2
 skipsdist = True
 
@@ -18,8 +18,6 @@ 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
   pip install -r requirements.txt
   python manage.py test qinling_dashboard.test --settings=qinling_dashboard.test.settings