Update tox.ini

* Move basepython to [testenv] which is a place to define common things
* Move upper-constraints from install_command to deps
* Update the min version to 2.0 to allow http links in deps
* Cleanup unused tox envs ('cover' and 'debug')
* Drop unnecessary envs from envlist (only pep8 is used now)

Change-Id: Ie915a93b824331407a48affcbd31076d6a7cccfa

tox.ini

@@ -1,39 +1,23 @@
 [tox]
-minversion = 1.6
-envlist = py34,py27,pypy,pep8
+minversion = 2.0
+envlist = pep8
 skipsdist = True
 
 [testenv]
+basepython = python3
 usedevelop = True
-install_command = pip install -U {opts} {packages}
-setenv =
-   VIRTUAL_ENV={envdir}
-deps = -r{toxinidir}/test-requirements.txt
+deps =
+  -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -r{toxinidir}/requirements.txt
+  -r{toxinidir}/test-requirements.txt
 commands = stestr run --slowest {posargs}
 
 [testenv:pep8]
-basepython = python3
 commands = flake8
 
 [testenv:venv]
-basepython = python3
 commands = {posargs}
 
-[testenv:cover]
-basepython = python3
-setenv =
-   {[testenv]setenv}
-   PYTHON=coverage run --source tempest_horizon --parallel-mode
-commands =
-   stestr run {posargs}
-   coverage combine
-   coverage html -d cover
-   coverage xml -o cover/coverage.xml
-
-[testenv:debug]
-basepython = python3
-commands = oslo_debug_helper {posargs}
-
 [flake8]
 show-source = True
 # W504 line break after binary operator