Fix issue in getDefaultReplicationPolicy.
Previously, under some circumstances, a ReplicationPolicy was created but was missing the optional replicationAllowed and numReplicas attributes. These are critical to the policy, so ensure they are set to sensible defaults even when property parsing errors and other issues might skip them. Added a test of this method, but I note that other methods in SystemMetadataFactory do not have unit tests and should be added if they are modified.