enable F812 check for flake8

The change removes the shadowed variable in list comprehensions and
enables the F812 check

Change-Id: If7b4d6f198795565fe0d8e7524b73555e310cd14
Partial-Bug: 1329017

neutron/services/vpn/device_drivers/ipsec.py

@@ -692,9 +692,7 @@ class IPsecDriver(device_drivers.DeviceDriver):
 
         # Delete any IPSec processes running
         # VPN that do not have an associated router.
-        process_ids = [process_id
-                       for process_id in self.processes
-                       if process_id not in router_ids]
+        process_ids = [pid for pid in self.processes if pid not in router_ids]
         for process_id in process_ids:
             self.destroy_router(process_id)
         self.report_status(context)

tox.ini

@@ -65,7 +65,7 @@ commands = python setup.py build_sphinx
 # E128 continuation line under-indented for visual indent
 # E129 visually indented line with same indent as next logical line
 # E265 block comment should start with ‘# ‘
-# F812 list comprehension redefines name from line
+# H104 file contains nothing but comments
 # H237 module is removed in Python 3
 # H305 imports not grouped correctly
 # H307 like imports should be grouped together
@@ -74,7 +74,7 @@ commands = python setup.py build_sphinx
 # H405 multi line docstring summary not separated with an empty line
 # H904 Wrap long lines in parentheses instead of a backslash
 # TODO(marun) H404 multi line docstring should start with a summary
-ignore = E125,E126,E128,E129,E265,F812,H237,H305,H307,H401,H402,H404,H405,H904
+ignore = E125,E126,E128,E129,E265,H237,H305,H307,H401,H402,H404,H405,H904
 show-source = true
 builtins = _
 exclude = .venv,.git,.tox,dist,doc,*openstack/common*,*lib/python*,*egg,build,tools,.ropeproject,rally-scenarios