provider/template: store relative path in state
provider/template: store relative path in state

This makes template_file usage in modules portable.

hashicorp/terraform-provider-template,btobolaski/terraform-provider-linode,terraform-providers/terraform-provider-template,terraform-providers/terraform-provider-template,btobolaski/terraform-provider-linode,terraform-providers/terraform-provider-template,hashicorp/terraform-provider-template,btobolaski/terraform-provider-linode,hashicorp/terraform-provider-template