Add check for makemessages in tox.ini PEP8

This commit adds a check of extracting message catalogs to
tox PEP8 check. It is useful to avoid a bug like bug 1379054.

It also fixes a warning "WARNING:root:No local_settings file found."
in run_tests.sh --makemessages and --compilemessages by specifying
Django settings.

Closes-Bug: #1379507

Change-Id: I5541e28a87d48601202bf8180a18eadb5a91770b
Merge "Add check for makemessages in tox.ini PEP8"