add parameter server launch

[guru4elephant]

test=develop

python -m distributed.launch_ps --worker_num 2 --server_num 2 trainer.py

When a user uses Paddle, he or she may use PaddleCloudRoleMaker which is very easy to local current processes' role. This pr mainly focuses on launch a parameter server job with worker_num and server_num configuration.
Example code is as follows:

import paddle.fluid.incubate.fleet.base.role_maker as role_maker
from paddle.fluid.incubate.fleet.parameter_server.distribute_transpiler import fleet

role = role_maker.PaddleCloudRoleMaker()
fleet.init(role)

if fleet.is_server():
    fleet.init_server()
    fleet.run_server()

elif fleet.is_worker():
    fleet.init_worker()
    exe.run(fleet.startup_program)

[seiriosPlus]

Delete ..

[seiriosPlus]

why copy twice

[seiriosPlus]

LGTM