Use new syntax for default HS/IS in Riot config.

This change makes no difference in itself, but is a pre-requisite for
adding the 'server name' config, and the old syntax is deprecated.

roles/matrix-riot-web/templates/config.json.j2

@@ -1,6 +1,12 @@
 {
-	"default_hs_url": {{ matrix_riot_web_default_hs_url|string|to_json }},
-	"default_is_url": {{ matrix_riot_web_default_is_url|string|to_json }},
+	"default_server_config": {
+		"m.homeserver": {
+			"base_url": {{ matrix_riot_web_default_hs_url|string|to_json }},
+		},
+		"m.identity_server": {
+			"base_url": {{ matrix_riot_web_default_is_url|string|to_json }}
+		}
+	},
 	"disable_custom_urls": {{ matrix_riot_web_disable_custom_urls|to_json }},
 	"disable_guests": {{ matrix_riot_web_disable_guests|to_json }},
 	"brand": {{ matrix_riot_web_brand|to_json }},