Support remapping arguments to python nodes using argparse

[dhood]

Without:

dhood@osrf-esteve:~/ros2_bouncy [ros2_bouncy]$ ros2 run demo_nodes_py listener_qos __ns:=/my_nsusage: listener_qos [-h] [--reliable] [-n NUMBER_OF_CYCLES]
listener_qos: error: unrecognized arguments: __ns:=/my_ns

With:

dhood@osrf-esteve:~/ros2_bouncy [ros2_bouncy]$ ros2 run demo_nodes_py talker_qos __ns:=/my_ns
[INFO] [my_ns.talker_qos]: Best effort talker
[INFO] [my_ns.talker_qos]: Publishing: "Hello World: 0"
dhood@osrf-esteve:~/ros2_bouncy [ros2_bouncy]$ $ ros2 rudemo_nodes_py listener_qos __ns:=/my_ns
[INFO] [my_ns.listener_qos]: Best effort listener
dhood@osrf-esteve:~/ros2_bouncy [ros2_bouncy]$ $ ros2 rulifecycle lifecycle_service_client_py.py get_state lc_talker __node:=my_client
[WARN] [my_client]: Unable to call service lc_talker/get_state

[wjwwood]

Is this preferable to https://docs.python.org/3.6/library/argparse.html#argparse.ArgumentParser.parse_known_args?

Also, there really should be an rclpy function to remove ros arguments from sys.argv before doing anything (including rclpy.init), so that you could remove them before giving the rest to argparse.

[dhood]

Thanks for mentioning the rclpy function that should exist one day, I forgot to mention this was meant to workaround that.

An advantage of REMAINDER over parse_known_args is that we can then pass just those extra arguments into rclpy (not that passing all of them is currently an issue). however, for the talker/listener_qos, I didn't actually pass it through, so the code is currently in a strange, mixed state. I'll followup.