Decouple the argument settings for common/ps.

[brightcoder01]

Currently the argument parser setting for PS is tightly coupled with common parameters. But there are a lot of parameters which are not required by the parameter server. We need decouple these two parts.

elasticdl/elasticdl/python/common/args.py

Lines 676 to 687 in 96227cc

	def parse_ps_args(ps_args=None):
	parser = argparse.ArgumentParser(description="ElasticDL PS")
	parser.add_argument(
	"--ps_id", help="ID unique to the PS", type=int, required=True
	)
	parser.add_argument(
	"--port", help="Port used by the PS pod", type=int, required=True
	)
	parser.add_argument("--master_addr", help="Master ip:port")
	add_common_params(parser)
	add_train_params(parser)