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: I5db260ef3cdd09a5cf6b87733db34b5ad830deab

tox.ini

@@ -26,6 +26,7 @@ commands =
  ostestr {posargs}
 
 [testenv:cover]
+basepython = python3
 commands =
   coverage erase
   python setup.py test --coverage --testr-args='{posargs}' \
@@ -33,27 +34,33 @@ commands =
   coverage report
 
 [testenv:debug]
+basepython = python3
 commands =
   oslo_debug_helper -t ./monasca_notification/tests {posargs}
 
 [testenv:pep8]
+basepython = python3
 commands =
   {[testenv:flake8]commands}
   {[testenv:bandit]commands}
 
 [testenv:venv]
+basepython = python3
 commands = {posargs}
 
 [testenv:flake8]
+basepython = python3
 commands =
   flake8 monasca_notification
   flake8 tests
 
 [testenv:bandit]
+basepython = python3
 commands =
   bandit -r monasca_notification -n5 -x monasca_notification/tests
 
 [testenv:genconfig]
+basepython = python3
 description = Generates an example of monasca-notification configuration file
 commands = oslo-config-generator \
   --config-file={toxinidir}/config-generator/notification.conf