Not to set OMP_NUM_THREADS

[wangkuiyi]

When I am reading @reyoung 's tutorial on performance tuning, I learned that we'd want to make sure that OpenMP doesn't automatically start multiple threads by setting the environment variable OMP_NUM_THREADS=1 on the command line.

However, I also noticed that our Python source code will change the environment variable:

Paddle/python/paddle/v2/__init__.py

Lines 114 to 115 in 1e6f85e

	set_env("OMP_NUM_THREADS", threads)
	set_env("MKL_NUM_THREADS", threads)

It seems that we should change the above code so that it doesn't change the value of the environment variable if it had been set.