Gets a "running" number represented by the cluster info.
             Some examples:
            
             1. NumberOfInstances=2, numberOfBackups=0, instanceId=1: RunningNumber = 0.
             2. NumberOfInstances=2, numberOfBackups=0, instanceId=2: RunningNumber = 1.
             3. NumberOfInstances=2, numberOfBackups=1, instanceId=1, backupId=0: RunningNumber = 0.
             4. NumberOfInstances=2, numberOfBackups=1, instanceId=1, backupId=1: RunningNumber = 1.
             5. NumberOfInstances=2, numberOfBackups=1, instanceId=2, backupId=0: RunningNumber = 2.
             6. NumberOfInstances=2, numberOfBackups=1, instanceId=2, backupId=1: RunningNumber = 3.
             
