Cleaner reporting

[cbeltranbird]

This PR aims to enhance the capabilities of the reporting command.

• Sets a new MINIMUM_REPORT_SEVERITY that complements the slack MINIMUM_ALERT_SEVERITY variable. This variable is defined in cdk_app.yaml and defaults to INFO. This is useful for accounts with a large number of INFO-level findings. (ex: IAM_LINTER)
• Enables detailed error reporting in Slack. Currently, non-blocking API errors will appear in collect logs and in Cloudwatch. This change allows per-account error counts to be presented. If no errors are detected, previous output message behavior is maintained.
• Suppresses the 1000's of cloudwatch log lines that come from the upload/sync actions from ECS to S3.
• Fixes the counts-of-findings-by-account graph in report.html
• Clarifies (but does not alter) the existing default minimum severity filter for the report command.
• Updates KNOWN_BAD_POLICIES to reference the new, safe V3 policy for AmazonMachineLearningRoleforRedshiftDataSourceV3
• audit_s3_block_policy() will emit account_name in Finding() to quickly root cause the offending account.
• EC2_IMDSV2_NOT_ENFORCED now provides the current MetaData Options to measure against. Also provides the ssh_key and instance ID, to assist with finding resources and determining risk.
• SG_CIDR_OVERLAPS and SG_LARGE_CIDR attempt to provide Security Group descriptive information.

[0xdabbad00]

Thank you! These are great. Nice work finding these.

LGTM. This can be merged @steiza