settings: Also define SERVER_EMAIL for email logs

settings: Also define SERVER_EMAIL for email logs

Django can send emails to admins on 500 HTTP errors when DEBUG is false.
That looks handy. That mechanism uses SERVER_EMAIL for the From:
address, which defaults to root@localhost and can cause problems in the
email delivery.

Use the same address than DEFAULT_FROM_EMAIL by default.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Acked-by: Stephen Finucane <stephen.finucane@intel.com>