Synnefo

snf-django-lib: Fix proxy issue

The ``X-Forwarded-Host`` header should not be forwared to the target
because django appends its value to the Host header and
results in validate_host() failure.

snf-django: Improve headers handling in common proxy util

- Avoid proxying Connection, Host and Cookie headers
- Cleanup non headers keys (e.g. wsgi.*)
- Set X-Forwarded-For header

snf-django-lib: provide proxy functionality