callbacks: Add ntfy callback plugin

This plugin sends a notification using _ntfy_ whenever a playbook
fails.  This will be useful especially for automated deployments when
the playbook was not launched manually.

.gitignore

@@ -5,3 +5,5 @@
 /victoria-metrics-*.tar.gz
 /victoria-metrics-*/
 /tmp/
+__pycache__/
+*.py[co]