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: Ie0015353ec57b2daa9ff626961c142289ac18069

requirements.txt

@@ -10,7 +10,7 @@
 pbr!=2.1.0,>=2.0.0 # Apache-2.0
 
 # Horizon Core Requirements
-horizon>=14.0.0.0b3  # Apache-2.0
+horizon>=17.1.0  # Apache-2.0
 openstacksdk>=0.26.0
 python-masakariclient>=5.4.0
 PyYAML>=3.12 # MIT

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = pep8,py27,py36,py37,py3-{dj111,dj22}
+envlist = pep8,py36,py37
 minversion = 3.1.1
 skipsdist = True
 ignore_basepython_conflict = True
@@ -20,8 +20,6 @@ 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 {posargs} --settings=masakaridashboard.test.settings --verbosity 2
 
 [testenv:pep8]