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: I6e73db41d64343dc76ef33bed4638c93b131bd88

tox.ini

@@ -21,16 +21,20 @@ commands =
     stestr --test-path ./patrole_tempest_plugin/tests/unit run {posargs}
 
 [testenv:pep8]
+basepython = python3
 commands = flake8 {posargs}
            check-uuid --package patrole_tempest_plugin.tests.api
 
 [testenv:uuidgen]
+basepython = python3
 commands = check-uuid --package patrole_tempest_plugin.tests.api --fix
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:cover]
+basepython = python3
 commands = rm -rf *.pyc
            rm -rf cover
            rm -f .coverage
@@ -46,6 +50,7 @@ whitelist_externals = nosetests
                       rm
 
 [testenv:docs]
+basepython = python3
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
   -r{toxinidir}/requirements.txt
@@ -56,6 +61,7 @@ commands =
 whitelist_externals = rm
 
 [testenv:releasenotes]
+basepython = python3
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
   -r{toxinidir}/requirements.txt
@@ -66,9 +72,11 @@ commands =
 whitelist_externals = rm
 
 [testenv:debug]
+basepython = python3
 commands = oslo_debug_helper -t patrole_tempest_plugin/tests {posargs}
 
 [testenv:genconfig]
+basepython = python3
 commands = oslo-config-generator --config-file etc/config-generator.patrole.conf
 
 [flake8]