htools: add optional limit to allocation count

Currently, the tieredAlloc/iterateAlloc functions will not return
until the allocation fails; this means unit-testing their
functionality (e.g. that an instance can be allocated) is slow, since
they will allocate all possible instances.

This patch adds an optional limit that allows allocation to return
early; this makes the cluster unittests twice as fast.

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