Lack of retention for log groups

[jsf9k]

          Hi Team, one potential medium severity issue at terraform/ files bod_vpc_flow_logs.tf, cyhy_vpc_flow_logs.tf, and mgmt_vpc_flow_logs.tf each file at line 43, aws-cloudwatch-log-group-no-retention. The AWS CloudWatch Log Group has no retention. Missing retention in log groups can cause losing important event information.

Originally posted by @dv4harr10 in #749 (comment)