Introduce flake8-import-order

This commit introduces flake8-import-order to check import orders
automatically. We shouldn't do it manually.

Depends-On: Ia2140f5566ce169b076ffa5c9ebfbdbfc41e5bed
Change-Id: Iff75ebec42b295870edd1c04001adfb17460a682
diff --git a/test-requirements.txt b/test-requirements.txt
index 475fb16..f7d63a8 100644
--- a/test-requirements.txt
+++ b/test-requirements.txt
@@ -9,3 +9,4 @@
 mock>=2.0 # BSD
 coverage>=4.0 # Apache-2.0
 oslotest>=1.10.0 # Apache-2.0
+flake8-import-order==0.11 # LGPLv3