Fix incompatibility with OVN-based clouds

[tschuettig]

Changes:

• add config to L3 client
• overwrite allowed address pairs after provisioning and deletion of L3 port
• periodically (currently every 5 minutes) overwrite allowed address pair to ensure correct setup after an agent was down
• move openstack config into separate file to avoid circular imports
• add wrappers to Portclient for better testability

The changes can be tested using the branch tschuettig/issue-659-lbaas from the yaook/k8s repository. (MR into sstrk/issue-659 is pending)

Fixes #53

[tschuettig]

@horazont Hope this is better now, if something could be improved let me know.