feat(fargate): set cloudwatch logs retention policy

[bernardobridge]

Description

Follow up of #2434

This sets retention policy for Cloudwatch logs. It sets an interval with a max number of retries to set the policy, so this isn't a blocking step and doesn't add additional time to the test unless something goes wrong.

Things that need review:

• Wasn't sure what was the ideal LOGGROUP_RETENTION_DAYS time should be.. Technically this is a change that could break workflows, if people are expecting a certain retention period. I put 6 months for now, but let's decide on the ideal time.
• I exposed the process.env.ARTILLERY_LOGGROUP_RETENTION environment variable so that this can be overridden. We may want this to be exposed in some other way, but for now it felt like a reasonable way to do it.

Pre-merge checklist

• Does this require an update to the docs? Yes
• Does this require a changelog entry? Yes, and should highlight that it will update existing tests

[hassy]

6 months sounds very reasonable. LGTM!