Try harder to set a default gateway

If we are not given an explicit v4 gateway, fall back to the prior
behavior of using the v4 subnet setting on the external network, but
only use one value.

Change-Id: Ia379833797039fb07d92f6fe496f07d6b0721bbc
remove pep8 section from tox.ini

We use flake8 in the style section, so we don't need a separate pep8
section.

Change-Id: Id61efc97bc32d47157d2f638557402a03ed8885c
Merge pull request #40 from dhellmann/neutron-ip-race-condition

Try harder to set a default gateway