Fix pep8 error and enable flake8 options

This Patch fixes flake8 error and enables this test
for all new patches to tacker-horizon.

H701 Empty localization string

see
https://github.com/openstack-dev/hacking/blob/master/
hacking/checks/localization.py#L109

see OpenStack Style Guidelines
http://docs.openstack.org/developer/hacking/

This Patch enables two flake8 options:
show-source
max-complexity

show-source adds output with source code for each error

see for max-complexity
https://flake8.readthedocs.org/en/latest/

Change-Id: I337846738d53488093d324ebfe1c8b21ce3681f6
Closes-Bug: #1531680

tacker_horizon/openstack_dashboard/dashboards/nfv/vnfmanager/forms.py

@@ -147,4 +147,5 @@ class DeployVNF(forms.SelfHandlingForm):
             return True
         except Exception as e:
             exceptions.handle(request,
-                              _(e.message))
+                              _('Failed to create VNF: %s') %
+                              e.message)

tox.ini

@@ -47,5 +47,5 @@ setenv = NOSE_WITH_COVERAGE=1
 commands = {posargs}
 
 [flake8]
-# H701 Empty localization string
-ignore = H701
+show-source = True
+max-complexity = 20