In tired allocation, hspace shrinks that resource of the instance
next, that causes failure on most nodes. While, this is not a bad
strategy in general, it can lead hspace into a dead end if for a large
number of nodes a particular resource blocks any further allocation of
policy compliant instances. So we improve the heuristics in that it
chooses to shrink such a resource next where by shrinking only this
resource a valid allocation can be made, if such a resource
exists. This improves the results in some cases, while still keeping
the computational complexity of the algorithm low.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>