ENYO-1369: set CORS HTTP headers & allow custom user-set HTTP headers

Tested on OSX/Canary.

- use conservative CORS headers by default & allow origin override (hosting behind an SRP)
- all Ares-wide user-defined custom headers
- remove per-service CORS settings

Enyo-DCO-1.1-Signed-off-by: Francois-Xavier KOWALSKI <francois-xavier.kowalski@hp.com>