Enable flag to bypass logs with retention already set

[mcraig88]

Enable 'overwrite' flag, basically do you want to overwrite an existing retention period. This would be helpful in case some logs already have a policy of say 30 days, but for buckets with no policy defined we want a max of 180 days retention.

https://github.com/swoodford/aws/blob/fd6fc2b3fecaf52773d1680a46e9d934ac9bb20b/cloudwatch-logs-retention-policy.sh#L10

[dineshcsdev]

Hi 👋 I'd like to work on this issue. I can implement an --overwrite flag that forces the script to reset existing retention periods when desired. Please let me know if it's okay to proceed.