Add support for session affinity

[tim-smart]

This project is awesome! Looking at using it to balance a kubernetes cluster - we were looking at vulcand but it doesn't quite have the feature set we are looking for. Namely websocket support and session affinity.

Would it be possible to support session affinity (sticky sessions) with this project? Take a look at this blog post for more information: http://blog.haproxy.com/2012/03/29/load-balancing-affinity-persistence-sticky-sessions-what-you-need-to-know/

All that remains is a service to sync the kubernetes api to the yoda etcd structure...

[sukrit007]

@tim-smart : Sorry missed your issue earlier. Though we design our apps to be stateless / session-less , I can see the reasoning of having sticky sessions as well. If you want to throw in a PR with ability to customize it per upstream, I can accept the change (Note: default value should be off).

On separate note, we might be also adding logic to customize the haproxy template file (from Amazon S3 has default source) , so that you can clone default template and turn it on for all your apps.

See: #20