Cleanup tox.ini: Remove obsolete constraints

Remove old and unused constraints environments from tox.ini. Those
have never been used. Use standard environments as default list.

Note that the repo has in the past not used constraints in OpenStack
CI, this change keeps the status quo.

For more information about constraints see:
http://lists.openstack.org/pipermail/openstack-dev/2016-August/101474.html

Change-Id: I9750ef4935b2ee1609545358fc5d4f590c1d2830

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 minversion = 1.8
-envlist = py34-constraints,py27-constraints,pypy-constraints,pep8-constraints
+envlist = py34,py27,pypy,pep8
 skipsdist = True
 
 [testenv]
@@ -13,44 +13,21 @@ deps = -r{toxinidir}/test-requirements.txt
        -r{toxinidir}/requirements.txt
 commands = python setup.py testr --slowest --testr-args='{posargs}'
 
-#[testenv:common-constraints]
-#install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} {opts} {packages}
-
 [testenv:pep8]
 commands = flake8 {posargs}
 
-#[testenv:pep8-constraints]
-#install_command = {[testenv:common-constraints]install_command}
-#commands = flake8 {posargs}
-
 [testenv:venv]
 commands = {posargs}
 
-#[testenv:venv-constraints]
-#install_command = {[testenv:common-constraints]install_command}
-#commands = {posargs}
-
 [testenv:cover]
 commands = python setup.py testr --coverage --testr-args='{posargs}'
 
-#[testenv:cover-constraints]
-#install_command = {[testenv:common-constraints]install_command}
-#commands = python setup.py testr --coverage --testr-args='{posargs}'
-
 [testenv:docs]
 commands = python setup.py build_sphinx
 
-#[testenv:docs-constraints]
-#install_command = {[testenv:common-constraints]install_command}
-#commands = python setup.py build_sphinx
-
 [testenv:debug]
 commands = oslo_debug_helper {posargs}
 
-#[testenv:debug-constraints]
-#install_command = {[testenv:common-constraints]install_command}
-#commands = oslo_debug_helper {posargs}
-
 [flake8]
 # E123, E125 skipped as they are invalid PEP-8.