add: allow existing bucket names as array

[berkguzel]

Fixes #275

This PR allows you to apply terraform code one time to scan several existing s3 buckets.

existing.sh script will ask you to provide existing s3 bucket names in a format like below.

 Bucket names: s3-bucket-protection-test s3-bucket-protection-test-test  

[ryanjpayne]

a better approach would be to use modules. one module for infra, one module to set up bucket. infra module is only called once, while the bucket module is called foreach and everytime a bucket needs to be added going forward