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