haproxy: Allow configuring the default balancing algorithm
Use source balancing for haproxy in Crowbar

This is what is recommended in the HA guide for OpenStack. And in the
end, it's fine with us because we are less interested in load balancing
than in making sure that everything works perfectly all the time.
Merge pull request #89 from vuntz/haproxy-balance

Use source balancing for haproxy