Extend arena to support for training dataset or model storage management

[wsxiaozhang]

It should be helpful for data scientists to use command like "arena create data imagenet-full" to create, index and manage different training datasets for different training jobs.
Then when use arena submit training job, the "imagenet-full" can be passed in as parameter "data" directly.
The data actually can be a pointer to specific PVC or hdfs path, etc. It's easy to use 'data' to manage and record which training datasets to load, while needn't care about which storage backend is used for persistent.