Switch to SSM for AMI data source, update vars and docs

[dkeightley]

• Switch to SSM for consistent source of AMI IDs in every region
• Update variables and tfvars examples
• Update docs

[glovecchi0]

Tried AWS/k3s -->

cat terraform.tfvars
prefix                = "glovecchio"
aws_region            = "eu-west-1"
server_instance_count = 1
worker_instance_count = 1
rancher_password      = "Mypassword.1234"

It worked well.

[glovecchi0]

It worked well in the ap-south-2 region, too.