GenerateSetupKey pins a one-off key at a single use and ignores whatever
usage_limit the request asked for. A caller that asks for five gets a key
that works once, is told the creation succeeded, and finds out only by
using it.
Terraform found this the hard way: the provider sends the usage limit it
planned, the server stores a different one, the next refresh writes the
server's value into state, and from then on every plan sees a change on an
attribute that forces replacement. Adding a group to a one-off setup key
destroyed the key and issued a new secret in its place.
Values above 1 are now refused with a message naming the reusable type,
which is what a caller asking for more than one use wants. 0 is still
accepted: usage_limit is a required field with no null in it, so a caller
with nothing to say about the limit has no way to say that except by
sending 0, and refusing it would break every such client.
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