Change device_setter.py for choose ps device (#4008)

* Change device_setter.py for choose ps device

Add arguments "ps_devices" for replica_device_setter(), in order to choose ps decive .

* Change device_setter.py for choose ps device

Add arguments "ps_device_name" for replica_device_setter(), in order to choose ps decive.

* Update ps_job_name in replica_device_setter()

* Update ps_job_name in replica_device_setter()

* Add testPS2TasksWithCPUConstraint() 

Add testPS2TasksWithCPUConstraint() to test if ps_decive="/job:ps/cpu:0" can put all the variables on the CPU by default

* Modify code indentation