client: update node metadata to match app name and Nomad standard.

The metadata relates to the nodesim application, therefore this
change modifies the metadata prefix to match. Nomad also typically
uses dots for metadata partioning, so we use that format too.

main.go

@@ -245,10 +245,10 @@ func startClient(logger hclog.Logger, buildInfo *internalSimnode.BuildInfo, cfg
 		Reserved: &structs.Resources{},
 		Links:    map[string]string{},
 		Meta: map[string]string{
-			"simnode_id":      cfg.NodeNamePrefix,
-			"simnode_enabled": "true",
-			"simnode_version": buildInfo.Version,
-			"simnode_sum":     buildInfo.Sum,
+			"nodesim.id":      cfg.NodeNamePrefix,
+			"nodesim.enabled": "true",
+			"nodesim.version": buildInfo.Version,
+			"nodesim.sum":     buildInfo.Sum,
 		},
 		CSIControllerPlugins: make(map[string]*structs.CSIInfo),
 		CSINodePlugins:       make(map[string]*structs.CSIInfo),