Use flake8-import-order plugin

In reviews we usually check import grouping but it is boring.
By using flake8-import-order plugin, we can avoid this.
It enforces loose checking so it sounds good to use it.
This flake8 plugin is already used in tempest.

Note that flake8-import-order version is pinned to avoid unexpected
breakage of pep8 job.

Setup for unit tests of hacking rules is tweaked to disable
flake8-import-order checks. This extension assumes an actual file exists
and causes hacking rule unit tests.

application-import-names needs to be specified in tox.ini
to ensure application imports are placed after third-party imports.

Change-Id: If2067b950737ae7ebc65b2311955754be795758f

neutron_vpnaas_dashboard/test/settings.py

@@ -20,10 +20,11 @@ HORIZON_CONFIG.pop('dashboards', None)
 HORIZON_CONFIG.pop('default_dashboard', None)
 
 # Update the dashboards with neutron_vpnaas_dashboard
-import neutron_vpnaas_dashboard.enabled
 import openstack_dashboard.enabled
 from openstack_dashboard.utils import settings
 
+import neutron_vpnaas_dashboard.enabled
+
 settings.update_dashboards(
     [
         openstack_dashboard.enabled,

test-requirements.txt

@@ -7,6 +7,7 @@ hacking!=0.13.0,<0.14,>=0.12.0 # Apache-2.0
 
 coverage!=4.4,>=4.0 # Apache-2.0
 django-nose>=1.4.4 # BSD
+flake8-import-order==0.13 # LGPLv3
 mock>=2.0.0 # BSD
 mox3!=0.19.0,>=0.7.0 # Apache-2.0
 python-subunit>=0.0.18 # Apache-2.0/BSD

tox.ini

@@ -57,5 +57,9 @@ exclude = .venv,.git,.tox,dist,*lib/python*,*egg,build,node_modules
 enable-extensions=H203,H904
 max-complexity = 20
 
+# flake8-import-order configurations
+import-order-style = pep8
+application-import-names = neutron_vpnaas_dashboard
+
 [hacking]
 local-check-factory = horizon.hacking.checks.factory