Add two functions for checking cluster consistency

For now, we don't support instances allocated across two groups, and we
will reject such clusters. The isClusterConsistent function will return
a list of inconsistent instances, potentially allowing operation without
touch them (but only the rest).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Balazs Lecz <leczb@google.com>