Add environment variable to specify retention days of log group.

[raing3]

Change adds an environment variable "LOGSPOUT_CLOUDWATCH_RETENTION_DAYS" which allows the retention of the log group to be configured, either per container (using container labels/environment variables) or globally using a static value.

The Slack link in factbook.yaml (https://mdsol.slack.com/messages/devtools-public-forum) seems to not be publicly joinable as well. Even though the URL indicates it is maybe intended to be.

Let me know if anything should be changed.

Thanks!