Add support for setting protocol.

HAProxy driver will not support TCP, HTTP, or HEALTH mode
values via the new set_protocol() driver API method. Default
is HTTP. Invalid values will return a failure.

libra/worker/drivers/base.py

@@ -38,6 +38,10 @@ class LoadBalancerDriver(object):
         """ Add a server for which we will proxy. """
         raise NotImplementedError()
 
+    def set_protocol(self, protocol):
+        """ Set the protocol of the instance. """
+        raise NotImplementedError()
+
     def create(self):
         """ Create the load balancer. """
         raise NotImplementedError()