[haproxy] always add a cookie to the server

This makes it easier to do session persistence based on a cookie. Without an
accompanying `cookie` setting in the backend, this change has no effect, as
confirmed by Willie in this mailing list post:

http://marc.info/?l=haproxy&m=140312462018915&w=2
Merge pull request #76 from airbnb/add_cookie_options

[haproxy] always add a cookie to the server