set haproxy balance algorithm to roundrobin for horizon

Currently kolla-ansible sets haproxy balance algorithm to source for
horizon. We can set it to round-robin if the cache backend is memcached
or using the database as the session storage backend. So we can
distribute http requests evenly to all available horizon instances.

Closes-Bug: #1990523
Change-Id: I0721cadcf53d59947bc0db6a193bfafe49c41ad3