Reduce number of flake/pep8 ignored rules

Nova tox.ini has many ignored flake/pep8 rules. To make code more clean would be better to reduce it's numbers.

Currently, the ignored rules in nova/tox.ini is:

# E712 is ignored on purpose, since it is normal to use 'column == true'
# in sqlalchemy.
# H803 skipped on purpose per list discussion.
# E125 is deliberately excluded. See https://github.com/jcrocholl/pep8/issues/126
# The rest of the ignores are TODOs
# New from hacking 0.9: E129, E131, E265, E713, H407, H405, H904
# Stricter in hacking 0.9: F402, F812
# E251 Skipped due to https://github.com/jcrocholl/pep8/issues/301

ignore = E121,E122,E123,E124,E125,E129,E126,E127,E128,E131,E251,E265,E711,E712,E713,F811,F402,F812,H305,H307,H405,H803,H904