Introduce CONTENT_ORIGIN as a required setting
As discussed on the mailing list we need a new required setting because:
1. plugins like pulp_docker require this CONTENT_HOST to be set, but others may not, so this setting could become a dividing line for plugins that want it set versus those that don't.
2. Clients are semantically unsure how to handle responses for one Pulp server versus another. This setting changes the client's responsibilities (relative vs absolute URL handling).
3. Relative urls are not as usable as absolute urls.
Add a setting named CONTENT_ORIGIN that is required.