Add Python 3.8 env in Travis CI test matrix.
At present the pinning of flake8 disallows running of lint on
Python 3.8 systems.
Update flake8 ignore-list to ignore W504 instead of W503, the PEP
guidance is that either is ok, but there must be local consistency.
There are more occurences of binary operator before line-break
than after in this repository, and we have also chosen to ignore
W504 in most of our other repositories.
Providing a policy.d test for Octavia is a bit more complicated as the
obvious policy is just to prevent listing of load balancers;
unfortunately it doesn't work in the upstream, as the code doesn't check
the policy in octavia. However, "showing" an load balancer does involve
a policy check and so the test is more complicated than 'normal' policy
tests in that a resource is created, then the policy override is checked
and then the resource is deleted.