Fixes#7975
This changes the InputMode for the CLI to always be:
InputModeProvider | InputModeVar | InputModeVarUnset
Which means:
* Ask for provider variables
* Ask for user variables _that are not already set_
The change is the latter point. Before, we'd only ask for variables if
zero were given. This forces the user to either have no variables set
via the CLI, env vars, tfvars or ALL variables, but no in between. As
reported in #7975, this isn't expected behavior.
The new change makes is so that unset variables are always asked for.
Users can retain the previous behavior by setting `-input=false`. This
would ensure that variables set by external sources cover all cases.
Mitchell Hashimoto