give get node/partition calls federation support

Add "cluster_name" field to node_info_t and partition_info_t data structure.
    It is filled in only when the cluster is part of a federation and
    SHOW_GLOBAL flag used.
Functions slurm_load_node() slurm_load_partitions() modified to show all
    nodes/partitions in a federation when the SHOW_GLOBAL flag is used.