Fix status report when key generation is inflight

This is a pain to test because it fails only when we are actively generating keys.

@oferrigni @marcparadise @sdelano @tylercloke @doubt72 @jmink
I agree with @marcparadise. As long as there's nothing else that calls `chef_keygen_cache:status()` then we have a return format from that function that the consumer can easily use, and the count of inflight here is perfect.