httpserver: Accept only TLSv1.2

SSLv3 is prone to a POODLE [1] attach, so we should not accept this
method.

[1] http://googleonlinesecurity.blogspot.fr/2014/10/this-poodle-bites-exploiting-ssl-30.html

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
httpserver: tests: ensure that SSLv3 is rejected

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>