Add support for `external image storage` via an S3 bucket

[egarbi]

Description:
Grafana can render the panel associated with the alert rule and include that in the notification.
Most Notification Channels require that this image be publicly accessible (Slack and PagerDuty for example).
In order to include images in alert notifications, Grafana can upload the image to an image store.
An s3 bucket con the proper permissions can be used to upload rendered imaged to be sent within notifications.
Tasks:

• Automate the creation of an s3 bucket via Cloudformation.
• Pass the proper environment variables to enable the feature.

More info here and here