Automatically launch a number of processes in the cluster

This ticket refers to the first milestone of SINGA-30.

NUsers do not need to provide a host file for each job. Singa will automatically calculated the required number of hosts from the cluster.conf, and generate a host list for the job.

We will add a new 'conf' dir under singa base path. In the future, this folder should contains all global settings, such as global host file, zookeeper settings, log directories...