Add app.wsgi to target of pep8

The app.wsgi file should be adjusted to meet pep8 standards
and apply pep8 check to app.wsgi automatically.

Change-Id: Ie9be25fb6bd290c65c25907309f482d736fb8a68

magnum/api/app.wsgi

@@ -14,7 +14,6 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-import os
 import sys
 
 from magnum.api import app as api_app

tox.ini

@@ -149,6 +149,7 @@ commands =
 # H106 Don’t put vim configuration in source files
 # H203 Use assertIs(Not)None to check for None
 # H904 Delay string interpolations at logging calls
+filename = *.py,app.wsgi
 enable-extensions = H106,H203,H904
 exclude = .venv,.git,.tox,dist,*lib/python*,*egg,build,tools,releasenotes