tieredAlloc: in case of FailN1, shrink memory
tieredAlloc: in case of FailN1, shrink memory

When computing tiered allocation statistics, the normal
step is to shrink the resource showing the most errors.
However, for some abstract resources, like N+1 redundancy,
there is no physical resource this concept refers to;
nevertheless, there is an underlying physical resource that
most likely causes this kind of failure. For N+1 redundancy,
the missing resource almost always is memory. So shrink
based on this assumption.

Signed-off-by: Klaus Aehlig <4ce51de36d9068cd7db2eec1151e0f3f95419008@google.com>
Reviewed-by: Petr Pudlak <9d19625f8ee825a2c398462ed335cbcf98f79c32@google.com>

leshchevds/ganeti,bitemyapp/ganeti,ganeti/ganeti,leshchevds/ganeti,andir/ganeti,bitemyapp/ganeti,mbakke/ganeti,ganeti/ganeti,andir/ganeti,ganeti/ganeti,mbakke/ganeti,onponomarev/ganeti,mbakke/ganeti,leshchevds/ganeti,onponomarev/ganeti,andir/ganeti