Make QuickCheck generator names more uniform

Due to lack of attention, we have two styles for generators of
arbitrary values: get* and gen* (e.g. getFQDN and genDiskIndices). In
order to make this more obvious that we deal with a function in the
Gen monad, let's rename all get* functions to gen*.

A few other simplifications were done on existing tests, switching to
helpers that were introduced later.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>