Disable the finding of python, swig related packages completely when WITH_PYTHON and WITH_SWIG_PY are set to OFF.

to create a gradient machine for inference using merged model with parameters
which is genearted by `paddle merge_model`.

* User must install Paddle python package before unittest.
* Or use docker to build Paddle

* Do not use `virtualenv` in Docker
* Split unittest into many target, make them parallel

and disable both when build doc and MacOS