Drop python 2.7 support and testing

OpenStack is dropping the py2.7 support in ussuri cycle.

tacker-horizon is ready with python 3 and ok to drop the
python 2.7 support.

Also this patch will change project templates to use horizon defined
templates to fix gate gate failures and update minimum tox version
in tox.ini file.
Complete discussion & schedule can be found in
- http://lists.openstack.org/pipermail/openstack-discuss/2019-October/010142.html
- https://etherpad.openstack.org/p/drop-python2-support

Ussuri Communtiy-wide goal - https://review.opendev.org/#/c/691178/

Change-Id: I7333f31aa8d836b35bd77f21b72217345599a936

.zuul.yaml

@@ -1,9 +1,8 @@
 - project:
     templates:
       - check-requirements
-      - openstack-lower-constraints-jobs
-      - openstack-python-jobs
-      - openstack-python3-train-jobs
+      - openstack-lower-constraints-jobs-horizon
+      - openstack-python3-ussuri-jobs-horizon
       - publish-openstack-docs-pti
     check:
       jobs:

releasenotes/notes/drop-py-2-7-55250f584936f88e.yaml

@@ -0,0 +1,6 @@
+---
+upgrade:
+  - |
+    Python 2.7 support has been dropped. Last release of tacker-horizon
+    to support python 2.7 is OpenStack Train. The minimum version of Python now
+    supported by tacker-horizon is Python 3.6.

setup.cfg

@@ -14,8 +14,6 @@ classifier =
     License :: OSI Approved :: Apache Software License
     Operating System :: POSIX :: Linux
     Programming Language :: Python
-    Programming Language :: Python :: 2
-    Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7

tox.ini

@@ -1,7 +1,8 @@
 [tox]
-envlist = py27,py37,pep8,docs
-minversion = 2.0
+envlist = py37,pep8,docs
+minversion = 3.1.1
 skipsdist = True
+ignore_basepython_conflict = True
 
 [testenv]
 basepython = python3
@@ -17,11 +18,6 @@ deps =
 commands =
   {envpython} {toxinidir}/manage.py test tacker_horizon --settings=tacker_horizon.test.settings {posargs}
 
-[testenv:py27]
-basepython = python2.7
-commands =
-  {envpython} {toxinidir}/manage.py test tacker_horizon --settings=tacker_horizon.test.settings {posargs}
-
 [testenv:py36]
 basepython = python3.6
 commands =