Adds ability to configure ZoneAwareLoadBalancing settings as config options (#480)

This adds ability to configure settings for zone aware load balancing settings as config options.
solves envoyproxy/envoy#1344

Changes : Added min_cluster_size and enabled_percentage to commonLbConfig which enables them to used directly in load_balancer_impl.cc.

Signed-off-by: sri kailash <sri.gandebathula@booking.com>

envoy/api/v2/cds.proto

@@ -365,6 +365,21 @@ message Cluster {
     // .. note::
     //   The specified percent will be truncated to the nearest 1%.
     core.Percent healthy_panic_threshold = 1;
+    message ZoneAwareLbConfig {
+      // [#not-implemented-hide:]
+      // Configures percentage of requests that will be considered for zone aware routing
+      // if zone aware routing is configured. If not specified, the default is 100%.
+      // * :ref:`runtime values <config_cluster_manager_cluster_runtime_zone_routing>`.
+      // * :ref:`Zone aware routing support <arch_overview_load_balancing_zone_aware_routing>`.
+      core.Percent routing_enabled = 1;
+      // [#not-implemented-hide:]
+      // Configures minimum upstream cluster size required for zone aware routing
+      // If upstream cluster size is less than specified, zone aware routing is not performed
+      // even if zone aware routing is configured. If not specified, the default is 6.
+      // * :ref:`runtime values <config_cluster_manager_cluster_runtime_zone_routing>`.
+      // * :ref:`Zone aware routing support <arch_overview_load_balancing_zone_aware_routing>`.
+      google.protobuf.UInt64Value min_cluster_size = 2;
+    }
   }
 
   // Common configuration for all load balancer implementations.