fix tox python3 overrides

We want to default to running all tox environments under python 3, so
set the basepython value in each environment.

We do not want to specify a minor version number, because we do not
want to have to update the file every time we upgrade python.

We do not want to set the override once in testenv, because that
breaks the more specific versions used in default environments like
py35 and py36.

Change-Id: Iceeaad2d6e31f3afeba5879ba3825caba437940d
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -27,6 +27,7 @@ commands =
   venv: {posargs}
 
 [testenv:py35]
+basepython = python3
 setenv =
   PYTHONUNBUFFERED = 1
   {[testenv]setenv}
@@ -58,6 +59,7 @@ commands =
   doc8 doc/source releasenotes/source releasenotes/notes
 
 [testenv:cover]
+basepython = python3
 commands =
   coverage erase
   coverage run {toxinidir}/manage.py test horizon --settings=horizon.test.settings {posargs}
@@ -67,6 +69,7 @@ commands =
   coverage html
 
 [testenv:selenium]
+basepython = python3
 setenv =
   {[testenv]setenv}
   WITH_SELENIUM=1
@@ -74,6 +77,7 @@ setenv =
 commands = {[unit_tests]commands}
 
 [testenv:selenium-headless]
+basepython = python3
 setenv =
   {[testenv]setenv}
   SELENIUM_HEADLESS=1
@@ -82,6 +86,7 @@ setenv =
 commands = {[unit_tests]commands}
 
 [testenv:selenium-phantomjs]
+basepython = python3
 setenv =
   {[testenv]setenv}
   SELENIUM_PHANTOMJS=1
@@ -102,6 +107,7 @@ basepython = python2.7
 commands = nosetests openstack_dashboard.test.integration_tests {posargs}
 
 [testenv:npm]
+basepython = python3
 passenv =
   HOME
   DISPLAY
@@ -111,6 +117,7 @@ commands =
   npm run {posargs:test}
 
 [testenv:tests_system_packages]
+basepython = python3
 # Provide an environment for system packagers that dont want anything from pip
 # Any extra deps needed for this env can be passed by setting TOX_EXTRA_DEPS
 sitepackages = True
@@ -122,6 +129,7 @@ commands =
   {[unit_tests]commands}
 
 [testenv:docs]
+basepython = python3
 # We need to install horizon dependecies to build module references
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
@@ -131,6 +139,7 @@ commands =
   sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:releasenotes]
+basepython = python3
 # There is no need to install horizon.
 usedevelop = False
 deps =
@@ -140,6 +149,7 @@ commands =
   sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
 
 [testenv:manage]
+basepython = python3
 # we don't need to install test-requirements.txt
 # modules in requirements.txt are installed due to usedevelop=True
 # so 'deps' can be empty.