Remove the requires of opencv-python.

[Xreki]

On some platform such as Nvidia DRIVE PX 2, we cannot install python wrapper of opencv by pip install opencv-python, which will return

$ pip install opencv-python
Collecting opencv-python
  Could not find a version that satisfies the requirement opencv-python (from versions: )
No matching distribution found for opencv-python

As a result, we cannot install python wrapper of PaddlePaddle (resolve #2305 ).
In fact, we can install opencv through other ways, such as apt-get install python-opencv.

[Xreki]

@qingqing01
As I disable the dependency of opencv-python, and Paddle will copy its own python, we cannot use apt-get install python-opencv to install cv2 to the new python. Thus, unit test test_v2_api will fail because of following error:

71: test test_image.py
71: E
71: ======================================================================
71: ERROR: test_resize_flip_chw (__main__.Image)
71: ----------------------------------------------------------------------
71: Traceback (most recent call last):
71:   File "test_image.py", line 23, in test_resize_flip_chw
71:     im = image.load_image('cat.jpg')
71:   File "/home/ubuntu/liuyiqun01/Paddle/python/paddle/v2/tests/.test_env/local/lib/python2.7/site-packages/paddle/v2/image.py", line 139, in load_image
71:     im = cv2.imread(file, flag)
71: AttributeError: 'NoneType' object has no attribute 'imread'
71: 
71: ----------------------------------------------------------------------
71: Ran 1 test in 0.001s
71: 
71: FAILED (errors=1)
1/1 Test #71: test_v2_api ......................***Failed  219.78 sec

The other two unit tests listed in #2305 , reader_tests and test_ploter, passed.

[qingqing01]

@Xreki Oh, I see. LGTM.