Need a better way to switch default program

[reyoung]

There are multiple program when we training a neural network using fluid API. They are:

1. The training program(s). There could be many training programs if we training a complex model, like GAN, RL, etc.
2. The testing program(s). Testing programs used to evaluate the accuracy or cost on test data set while training. We use an independent program while testing because there are some operators using a different equation when testing, such as dropout.

We need a better way to easily switch default program for layers in our API.

This issue will be done in two steps:

1. remove g_main_program/g_startup_program. Use default_main_program and default_startup_program instead.
2. Add API switch_program to change default programs.