A common reason to want to use `terraform plan` is to have a chance to
review and confirm a plan before running it.  If in fact that is the
only reason you are running plan, this new `terraform apply -auto-approve=false`
flag provides an easier alternative to

    P=$(mktemp -t plan)
    terraform refresh
    terraform plan -refresh=false -out=$P
    terraform apply $P
    rm $P

The flag defaults to true for now, but in a future version of Terraform it will
default to false.