allow optional namespace args

[rhaschke]

This fixes another regression introduced by #835: The newly introduced optional ns args are not declared as optional to python, thus resulting in errors like the following if omitted:

Traceback (most recent call last):
  File "/homes/rhaschke/src/ros/src/agni/agni-manipulation/agni_grasp_manager/scripts/grasp_manager.py", line 78, in __init__
    r = RobotInterface("robot_description")
Boost.Python.ArgumentError: Python argument types in
    RobotInterface.__init__(RobotInterface, str)
did not match C++ signature:
    __init__(_object*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >)

[gavanderhoorn]

I believe this was also reported by a user on ROS Answers: moveit_commander_cmdline.py Error.

[rhaschke]

@willcbaker Please review as you originally requested #835.
There is also another open issue pending with this: #835 (review)

[willcbaker]

@rhaschke looks good visually, still unable to test at this time

[v4hn]

picked to melodic