a) machine is a NAME NODE, fs.default.name=localhost:9000  // nothing has to be changed

b) machine is a DATA NODE, fs.default.name=Hostname or IP address of NameNode. 
// replace value at /opt/hadoop-0.15.3/conf/hadoop-site.xml
// modify /opt/hadoop-0.15.3/conf/masters - append Hostname or IP address of NameNode.
// modify /opt/hadoop-0.15.3/conf/slaves - ?

c) machine is a BACKUP NAME NODE, fs.default.name=Hostname or IP address of NameNode.
// replace value at /opt/hadoop-0.15.3/conf/hadoop-site.xml
// modify /opt/hadoop-0.15.3/conf/masters - append Hostname or IP address of NameNode.
// modify /opt/hadoop-0.15.3/conf/slaves - ?