Convert linters from tox to nox

This expands the noxfile to include the linters session and switches
the tox-linters job with the nox-linters job.

Change-Id: If5ff34aded40c66667eaa468baea10fb2658e2b3

.zuul.yaml

@@ -97,14 +97,14 @@
 - project:
     check:
       jobs:
-        - tox-linters
+        - nox-linters
         - zuul-nox-docs
         - zuul-operator-build-image
         - zuul-operator-functional-k8s:
             dependencies: zuul-operator-build-image
     gate:
       jobs:
-        - tox-linters
+        - nox-linters
         - zuul-nox-docs
         - zuul-operator-upload-image
         - zuul-operator-functional-k8s:

noxfile.py

@@ -45,3 +45,10 @@ def docs(session):
     session.install('-e', '.')
     session.run('sphinx-build', '-E', '-W', '-d', 'doc/build/doctrees',
                 '-b', 'html', 'doc/source/', 'doc/build/html')
+
+
+@nox.session(python='3')
+def linters(session):
+    set_standard_env_vars(session)
+    session.install('flake8')
+    session.run('flake8')