Alertmanager templates load doesn't support mutiple level files

[songjiayang]

cortextool alertmanager load ./tmp/rules.yaml ./tmp/fool.tmpl execution failed. Because cortex backend doesn't support ./tmp/fool.tmpl template name. Can we filepath.Base the config file name before post data to backend. related code commands/alerts.go#L129

templates[f] = string(tmpl)  => templates[filepath.Base(f)] = string(tmpl)