Allow GetMasterCandidateStats to ignore some nodes

This patch modifies ConfigWriter.GetMasterCandidateStats to allow it to
ignore some nodes in the calculation, so that we can use it to predict
cluster state without some nodes (which we know we will modify, and thus
we should not rely on their state).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Olivier Tharan <olive@google.com>