Update mnist_replica.py

1. Delete the redundant argument 'FLAGS.num_workers' 
2. Change:'num_gpu' to 'num_gpus', and support to use'/cpu:0' instead of '/gpu:#' when setting 'num_gpus=0' 
3. Combine the 'if FLAGS.sync_replicas' statement to `SyncReplicasOptimizer` block.
4. Add `FLAGS.job_name`, 'FLAGS.task_index' and 'FLAGS.num_gpus' available check. It will raise an exception if any of them is not set, or unknown