Add optional binding to haproxy config
Add optional binding to haproxy config

Updated haproxy.conf to use a binding named backend if one is available.  If not, it falls back to the original config values.  This allows haproxy to automatically add all members of a service group as backends.

Signed-off-by: Nolan Davidson <ndavidson@chef.io>

Pull request: #901
Approved by: smith

georgemarshall/habitat,nathenharvey/habitat,rsertelon/habitat,georgemarshall/habitat,habitat-sh/habitat,habitat-sh/habitat,habitat-sh/habitat,habitat-sh/habitat,rsertelon/habitat,nathenharvey/habitat,nathenharvey/habitat,georgemarshall/habitat,georgemarshall/habitat,nathenharvey/habitat,georgemarshall/habitat,habitat-sh/habitat,georgemarshall/habitat,nathenharvey/habitat,nathenharvey/habitat,rsertelon/habitat,georgemarshall/habitat,georgemarshall/habitat,rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,rsertelon/habitat,rsertelon/habitat