Use new header for client ip to fix bug 1025325

Change nginx to pass through headers with underscores
Change sign_agreement view to use HTTP_X_CLUSTER_CLIENT_IP with
HTTP_X_FORWARDED_FOR as fallback
Fix flake8 errors
Fix protected_asset tests and extend coverage

Fix flake8 errors
Fix test errors
Add test_process_request_with_cluster_client_ip
Remove unneeded test_process_request_without_proxy
Merge pull request #140 from jgmize/signed-agreement-client-ip

Use new header for client ip to fix bug 1025325