In `helper/schema` we already makes a distinction between `Default`
which is always applied and `InputDefault` which is displayed to the
user for an empty field.
But for variables we just have `Default` which is treated like
`InputDefault`. This changes it to _not_ prompt the user for a value
when the variable declaration includes a default.
Treating this as a UX bugfix and the "don't prompt for variables w/
defaults set" behavior as the originally expected behavior we were
failing to honor.
Added an already-passing test to verify and cover the `helper/schema`
behavior.
Perhaps down the road we can add a `input_default` attribute to
variables to allow similar behavior to `helper/schema` in variables, but
for now just sticking with the fix.
Fixes#2592
