Allow SECURE_PROXY_SSL_HEADER to be set via the environment.
Allow SECURE_PROXY_SSL_HEADER to be set via the environment.

Also only set USE_X_FORWARDED_HOST to True in prod.

Osmose/normandy,mozilla/normandy,mozilla/normandy,Osmose/normandy,mozilla/normandy,mozilla/normandy,Osmose/normandy,Osmose/normandy