Created by: kexinzhao

fixes #7221 (closed)

Put fetch_var_names and feed_var_names as repeated string fields in the ProgramDesc

To test this PR：

1. build paddle based on this pr
2. Use pip install -U /home/liuyiqun01/PaddlePaddle/Paddle/build_paddle/dist/opt/paddle/share/wheels/paddlepaddle_gpu-0.11.0-cp27-cp27mu-linux_x86_64.whl to install the latest Paddle and run python test_recognize_digits_mlp.py to train the model, then you will get the inference model in sub-directory recognize_digits_mlp.inference.model

After these steps, you can run the example using:

$ GLOG_v=3 ./paddle/inference/example --dirname=/home/liuyiqun01/PaddlePaddle/Paddle/python/paddle/v2/fluid/tests/book/recognize_digits_mlp.inference.model 

The output is similar to #7097