Terraform Core expects all variables to be set, but for some ancillary
commands it's fine for them to just be set to placeholders because the
variable values themselves are not key to the command's functionality
as long as the terraform.Context is still self-consistent.
For such commands, rather than prompting for interactive input for
required variables we'll just stub them out as unknowns to reflect that
they are placeholders for values that a user would normally need to
provide.
This achieves a similar effect to how these commands behaved before, but
without the tendency to produce a slightly invalid terraform.Context that
would fail in strange ways when asked to run certain operations.
Any command using meta.defaultFlagSet *might* occasionally exit before
the flag package's output got written. This caused flag error messages
to get lost. This PR discards the flag package output in favor of
directly returning the error to the end user.
Alisdair McDiarmid