updated the Nessus scanner Market id from tenable documentation

20230823

Update README.md

Fix Tenable Scanner user-data to allow upgrading of scanner servers

I added the aws_scanner key as directed by the Tenable support.

20200409: Default instance size changed

• Use coalesce instead of merge+ternary

This is mostly my opinion, but it results in cleaner code. Resolves #5

• Fix local.instance_tags. I broke it with the last commit.

• Remove user_data template

Switch to direct jsonencode() of relevant values inside of a map

• Default instance size changed

Tenable locked the AMI from running on instances with less than 16 GB