Integrated Alerting: Alert template .yaml is not recognized

User impact:
If user adds .yaml file to ia templates, it is ignored.

Steps to reproduce:
1. Add template to /srv/ia/templates with .yaml extension
2. Reload alert templates (list with reload, or create/edit another template)
3. See if user-defined alert template was returned.

Actual result:
Yaml file is not ignored.

Expected result:
Yaml file should be returned.

Workaround:
Rename .yaml to .yml