horizon: Configure stickiness with haproxy

[vuntz]

We use cookie-based stickiness for non-SSL, and we also expire the stickiness based on the session timeout. This should make the haproxy/horizon combo to work well, with each session sticking to one backend, thus avoiding potential issues where requests are spread between multiple backends.

Depends on https://github.com/crowbar/crowbar-ha/pull/179