Ensure LB policy set for locality-aware routing (CE)

`overprovisioningFactor` should be overridden with the expected value
(100,000) when there are multiple endpoint groups. Update code and
tests to enforce this.

This is an Enterprise feature. This commit represents the CE portions of
the change; tests are added in the corresponding `consul-enterprise`
change.

agent/xds/endpoints.go

@@ -880,13 +880,7 @@ func makeLoadAssignment(logger hclog.Logger, cfgSnap *proxycfg.ConfigSnapshot, c
 		Endpoints:   make([]*envoy_endpoint_v3.LocalityLbEndpoints, 0, len(endpointGroups)),
 	}
 
-	if len(endpointGroups) > 1 {
-		cla.Policy = &envoy_endpoint_v3.ClusterLoadAssignment_Policy{
-			// We choose such a large value here that the failover math should
-			// in effect not happen until zero instances are healthy.
-			OverprovisioningFactor: response.MakeUint32Value(100000),
-		}
-	}
+	setFullFailoverProvisioningFactor := len(endpointGroups) > 1
 
 	var priority uint32
 
@@ -897,6 +891,10 @@ func makeLoadAssignment(logger hclog.Logger, cfgSnap *proxycfg.ConfigSnapshot, c
 			continue
 		}
 
+		if len(endpointsByLocality) > 1 {
+			setFullFailoverProvisioningFactor = true
+		}
+
 		for _, endpoints := range endpointsByLocality {
 			es := make([]*envoy_endpoint_v3.LbEndpoint, 0, len(endpointGroup.Endpoints))
 
@@ -930,6 +928,14 @@ func makeLoadAssignment(logger hclog.Logger, cfgSnap *proxycfg.ConfigSnapshot, c
 		}
 	}
 
+	if setFullFailoverProvisioningFactor {
+		cla.Policy = &envoy_endpoint_v3.ClusterLoadAssignment_Policy{
+			// We choose such a large value here that the failover math should
+			// in effect not happen until zero instances are healthy.
+			OverprovisioningFactor: response.MakeUint32Value(100000),
+		}
+	}
+
 	return cla
 }