Fix issues with rendering RDS inventory as a table + EKS support

[aminland]

Currently the RDS inventory throws an error. It can't be rendered because the vpc_name and security_group fields are returning a list. Joining these into a single string seems to do the trick.

Additionally, it is much more useful for matching purposes to show the DB Identifier rather than the db name (DB Identifier is what is shown in RDS console).

Update: This PR now also includes the ability to report on EKS clusters.