dashboard: add ssl_forward support

Allow to configure Apache to forward HTTPS protocol in the headers.
Useful when activating SSL binding on HAproxy and not in Horizon.

Closes-bug #520
HAproxy: SSL ACL support

Create an ACL "is-ssl" to detect if destination port is 443 (HTTPS).
Closes-bug #520
HAproxy / Horizon: keep header proto if using SSL in binding

If using SSL forwarding for Horizon endpoint, keep the HTTPS in headers
with "X-Forwarded-Proto" parameter, using is-ssl ACL.

Closes-bug #520
horizon: fix setenv vhost config to use setenvif

Closes-bug #520

Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
Merge pull request #521 from enovance/bug/emilien/520

Horizon / SSL Forwarding: keep HTTPS in headers