Instance generators

This patch introduces and enhances generators for
instances:
- 'genInstWithNets' is split into the generation of an
  arbitrary instance and enhancing an instance with nets
- 'genInst' calls 'genInstWithNets' with an empty set
  of initial networks to provide a reasonable default
- the Arbitrary instance of 'Instance' uses now 'genDisks'
  to create instances with a reasonable set of disks

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>