Default alert manager templates get overwritten by the content of `message` field

[maulikpandey]

According to AWS documentation we can use templates to produce JSON based output from alert manager. But in practice the output of templates get overwritten by the content of the message field of the sns_configs.

We need the message field content for integrating with Pagerduty and Slack. We are currently over coming this limitation by using the templating language to create blocks in the message field.