Explanation/Reference:
When using automatic service migration, the Node Manager is required to get health monitoring information about the member servers involved, as follows:
* Consensus leasing - Node Manager must be running on every machine hosting managed servers within the cluster.
* Database leasing - Node Manager must be running on every machine hosting managed servers within the cluster only if pre/post-migration scripts are defined. If pre/post-migrations are not defined, then Node manager is not required.
Reference: https://docs.oracle.com/cd/E13222_01/wls/docs103/cluster/service_migration.html