Init() always gives `-force-copy` as an argument

[hiyosi]

Init() always gives -force-copy flag into the terraform init command.

https://github.com/hashicorp/terraform-exec/blob/v0.15.0/tfexec/init.go#L119

In my understanding, this means that the terraform always (re)initialization the backend with -migrate-state even if -reconfigure is given.

https://www.terraform.io/cli/commands/init#backend-initialization

I consider that Init() should remove -force-copy if it gives -reconfigure flag to the terraform command.