Add max-complexity to pep8 for Horizon

Picked this up from Keystone. It sounds like a good defense to have from
further making our code more complex.

The current highest complexity is 32, set the maximum complexity
for flake8 to 33. This should help to keep code simple and
maintainable. It should be possible to reduce the maximum
complexity to something closer to 20 with relatively small efforts.

openstack_dashboard.dashboards.identity.projects.workflows:
    'UpdateProject.handle' is complexity of 32
openstack_dashboard.dashboards.project.access_and_security.security_groups.forms:
    'AddRule.clean' is complexity of 20
openstack_dashboard.dashboards.project.instances.workflows.create_instance:
    'SetInstanceDetailsAction.clean' is complexity of 26
horizon.exceptions:
    'handle' is complexity of 24

Change-Id: I58cc20972aac221012f248e8135151b1243dae35
Merge "Add max-complexity to pep8 for Horizon"