In CanTieredAlloc test set IPolicy

In CanTieredAlloc test set IPolicy

Changing the test to allocate on nodes with a non-trivial
instance policy has two advantages.

- We test in a more realistic (and also more challenging)
  environment.

- Once the lower limit of the ipolicy is reached no more
  allocations are possible, thus having fewer shrinking
  rounds---and hence speeding up the test by a factor of 4.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Hrvoje Ribicic <riba@google.com>