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: I566890ccc021260b785190b6c40b96ae415f2eac

tox.ini

@@ -27,6 +27,7 @@ commands =
      doc8 -e '' security-threat-analysis
 
 [testenv:checkbuild]
+basepython = python3
 commands =
   # Build and copy RST Guides
   {toxinidir}/tools/build-all-rst.sh
@@ -34,6 +35,7 @@ commands =
   openstack-indexpage publish-docs
 
 [testenv:publishdocs]
+basepython = python3
 # Prepare all documents so that they can get published on
 # docs.openstack.org with just copying publish-docs/* over.
 commands =
@@ -41,10 +43,12 @@ commands =
   {toxinidir}/tools/build-all-rst.sh
 
 [testenv:checklang]
+basepython = python3
 whitelist_externals = doc-tools-check-languages
 commands = doc-tools-check-languages doc-tools-check-languages.conf test all
 
 [testenv:buildlang]
+basepython = python3
 # Run as "tox -e buildlang -- $LANG"
 whitelist_externals =
     doc-tools-check-languages
@@ -55,6 +59,7 @@ commands =
     bash -c "find security-guide -type f -regex '.*\.pot?' -print0|xargs -0 -n 1 msgfmt --check-format -o /dev/null"
 
 [testenv:publishlang]
+basepython = python3
 whitelist_externals = doc-tools-check-languages
 commands = doc-tools-check-languages doc-tools-check-languages.conf test all
 
@@ -65,6 +70,7 @@ commands =
   {toxinidir}/tools/build-all-rst.sh
 
 [testenv:generatepot-rst]
+basepython = python3
 # Generate POT files for translation, needs {posargs} like:
 # tox -e generatepot-rst -- security-guide
 commands = {toxinidir}/tools/generatepot-rst.sh {posargs}