Clean up hardcoded values in variables.tf, backend.tf

[halfb00t]

• Remove backend.tf, add it to .gitignore
• Remove hardcoded state bucket from terragrunt.hcl
• Remove critical hardcoded values from variables.tf and create example .tfvars file
• Remove zone_id for the domain, use data
• Fix local variables for templates
• Move all possible data to the SM secret