Fallback configuration for alertmanager (#545)
Fallback configuration for alertmanager (#545)

* Use a fallback configuration to avoid having no alertmanager for an instance

When we have no alertmanager at all for an instance, both alertmanager
and ruler will log 404s repeatedly for every alert.  This lets you
configure a fallback config to do something like hit a webhook.

* Create a Slack URL from the userID if url has magic value

* Refactor: extract am.transformConfig() function

also move magic string to a const

tomwilkie/cortex,tomwilkie/cortex,tomwilkie/cortex