Currently the way we generate nodes in some cases is by creating a
totally random node, then restricting the test based on whether the node
'size' (as defined by multiples of base unit) satifies some high/low
rules. This results in hard-to-satisfy conditions, so we change this
model to be able to specify node sizes directly in the generation
process, thus no longer needed post-creation filters.

This fixes prop_ClusterAllocBalance which before had at most 1-2
satisfiable tests.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>