Bug: tfexec to support multiple tfvars files

[findkim]

The terraform-exec library currently does not support executing Terraform commands with multiple .tfvars files (source). Consul NIA needs to pass at least 2 variable definition files, one for service info terraform.tfvars and one for dynamic user inputs from user configs task.variables_file.

Until hashicorp/terraform-exec#33 is resolved, user input variables will not work and cause an error during TF plan/apply. Running Consul NIA with a configured task.variables_file will result in a Terraform error along the lines of:

Error: "<variable>": required field is not set

Null values are not allowed for this attribute value.