Use the same style checking everywhere

Both the run_tests.sh and tox.ini using the same check_source.sh for
style and basic lint testing.

Adding a pyflakes based unused import test.

Removing 2 unused imports from
tempest/tests/network/test_network_basic_ops.py.

Change-Id: I5b4c8b945495751ac68fc64ed5ebf1cf441a5a31
diff --git a/tools/test-requires b/tools/test-requires
index b799dce..f701dab 100644
--- a/tools/test-requires
+++ b/tools/test-requires
@@ -2,3 +2,4 @@
 pylint==0.19
 #TODO(afazekas): ensure pg_config installed
 psycopg2
+pyflakes