flake8: Fix use of --select and --ignore.

flake8: Fix use of --select and --ignore.

The flake8 command evolved over a series of patches and now includes the
use of both --select and --ignore.  Unfortunately, this wasn't doing
what I thought.  The use of --select completely overrides what --ignore
does, meaning that we were only currently enforcing a small number of
warnings specified in --select.  This patch runs flake8 twice, once with
--select and once with --ignore to actually enforce the full desired
set of warnings.

No additional violations had been introduced, but I noticed this while
working on some other patches.

Signed-off-by: Russell Bryant <russell@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>