Using the recently-added genArbitrary, we can now implement Arbitrary
instances for even "huge" objects like Cluster, so let's use that to
implement entire ConfigData serialisation tests.

Note that, as we don't have yet proper types for some of the Params
fields, we have to cheat via FlexibleInstances and
TypeSynonymInstances, using either empty items or real arbitrary
values.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>