Update pep8 testing

Remove comment about H904 since H904 has been removed from hacking,
there's no need to fix it.
Remove E712 ignore since the code is clean.
Remove E711 ignore and fix the problems in manila/db/sqlalchemy/api.py to
make it pass.
Remove H304 ignore since the code is clean.

Change-Id: I4f9477a61d6f1ddde16b5ff9e387c76076c89d6e
Merge "Update pep8 testing"