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: I5ac3da11e140593733992d9d5243c5c44fb72fd6

requirements.txt

@@ -6,4 +6,4 @@ pbr>=1.6
 oslo.log>=3.30.0 # Apache-2.0
 python-swiftclient>=2.2.0
 python-troveclient>=1.2.0
-horizon>=14.0.0.0b3  # Apache-2.0
+horizon>=17.1.0  # Apache-2.0

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 3.1.1
-envlist = py37,py3-{dj111,dj22},pep8
+envlist = py37,pep8
 skipsdist = True
 ignore_basepython_conflict = True
 
@@ -14,8 +14,6 @@ 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:pep8]