Define missing identifier variable

roles/custom/matrix-bot-go-neb/defaults/main.yml

@@ -5,6 +5,8 @@
 
 matrix_bot_go_neb_enabled: true
 
+matrix_bot_go_neb_identifier: 'go-neb-bot'
+
 matrix_bot_go_neb_version: latest
 
 matrix_bot_go_neb_scheme: https