Handle nodes without local storage

Our current cluster model is based on per-node storage, and as such we
don't handle shared disk templates. Until we can improve the model,
let's handle nodes without local storage better: in order to still
compute a valid cluster score, we need to handle nodes with tDsk == 0.

For this, we abstract the in-line computation of pDsk (which is used
in the cluster score) to a separate function which, for tDsk == 0,
returns instead 1 (i.e. disk fully used).

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