Add a test for hspace to respect instance policy

Add a test for hspace to respect instance policy

In this example, the instance policy for disks prevents allocation
of more than one instance per node. The test verifies that tiered
allocation still respects this and doesn't attempt to place policy
violating instances on the cluster.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Thomas Thrainer <thomasth@google.com>