Use const for host topology name

probe/kubernetes/reporter.go

@@ -722,7 +722,7 @@ func (r *Reporter) hostTopology() (report.Topology, error) {
 		}
 		result.AddNode(
 			report.MakeNode(report.MakeHostNodeID(n.Name)).
-				WithTopology("host").
+				WithTopology(report.Host).
 				WithLatestActiveControls(activeControl),
 		)
 	}

report/node.go

@@ -230,7 +230,7 @@ func (n Node) Merge(other Node) Node {
 	}
 
 	// Special case to merge controls from two different probes.
-	if topology == "host" {
+	if topology == Host {
 		newNode = newNode.MergeActiveControls(other)
 	}