(#22233) Warn but do not fail if find gce facts without json lib

Previous patch to GCE would fail outright if there were GCE facts to
parse, but we had no JSON library.  It is better if Facter is allowed to
gather what facts it can rather than failing outright, so this change
just warns that we cannot load GCE facts due to missing JSON gem,
similar to how we warn if there is a problem parsing the GCE facts.

Also fixes up specs a little for consistency and to remove unused
references.