Add (optional) fallback nuxeo.virtual.host parameter in nuxeo.conf

When Nuxeo is used with a HTTPS URL, it currently requires a nuxeo-virtual-host HTTP request header to be added to construct redirect URLs correctly.

However, some reverse proxies don't allow you to add HTTP request headers (eg Amazon ELB).
Currently, that means you have to add another level of reverse-proxy just to add the header.

With a nuxeo.virtual.host parameter in nuxeo.conf, this would be used as a fallback when the header is missing (but wouldn't permit the same Nuxeo to be accessed with different URLs).