Fixes#1409
Resource set hash calculation is a bit of a devil's bargain when it
comes to optional, computed attributes.
If you omit the optional, computed attribute from the hash function,
changing it in an existing config is not properly detected.
If you include the optional, computed attribute in the hash and do not
specify a value for it in the config, then you'll end up with a
perpetual, unresolvable diff.
We'll need to think about how to get the best of both worlds, here, but
for now I'm switching us to the latter and documenting the fact that
changing these attributes requires manual `terraform taint` to apply.
34c7bbcf4d