Fix exported json graphs' keys
 * keys of graph_name are duplicated for different service/section graphs
 * graph keys should be unique
   * duplicated key name are overwritten, and it make json broken
 * full-path service/section/graph_name keys are unique
   * this fix breaks compatibility of exported json keys
Merge pull request #52 from tagomoris/safe_json_xport

Fix exported json graphs' keys