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.

aws/cloudwatch-logs-retention-policy.sh

Line 10 in fd6fc2b