SSL: added ability to set keys used for Session Tickets (RFC5077).

SSL: added ability to set keys used for Session Tickets (RFC5077).

In order to support key rollover, ssl_session_ticket_key can be defined
multiple times. The first key will be used to issue and resume Session
Tickets, while the rest will be used only to resume them.

    ssl_session_ticket_key  session_tickets/current.key;
    ssl_session_ticket_key  session_tickets/prev-1h.key;
    ssl_session_ticket_key  session_tickets/prev-2h.key;

Please note that nginx supports Session Tickets even without explicit
configuration of the keys and this feature should be only used in setups
where SSL traffic is distributed across multiple nginx servers.

Signed-off-by: Piotr Sikora <piotr@cloudflare.com>