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: Iba67ef474d087eb2a3d60c2d44a2084ee7e566c3

tox.ini

@@ -2,6 +2,7 @@
 envlist = py27,pep8
 
 [testenv]
+basepython = python3
 deps = -r{toxinidir}/test-requirements.txt
 commands = nosetests {posargs}
 sitepackages = False
@@ -9,12 +10,15 @@ sitepackages = False
 [tox:jenkins]
 
 [testenv:cover]
+basepython = python3
 setenv = NOSE_WITH_COVERAGE=1
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:pep8]
+basepython = python3
 deps = flake8
 commands = flake8 setup.py astara_router
 
@@ -25,8 +29,10 @@ ignore =
 builtins = _
 
 [testenv:build_image]
+basepython = python3
 deps = diskimage-builder
 commands = bash -xec "./scripts/build_dib_appliance.sh {posargs}"
 
 [testenv:releasenotes]
+basepython = python3
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html