Add Python helpers to make life easier.

- Helper for getting all active controller managers.

- Helper for determining if a namespace contains the controller manager ROS API.

- Helpers for filtering the output of the 'list_controllers' service by
  type, name, state, hardware_interface and claimed resources.

Resolved conflicts:

	controller_manager_msgs/CMakeLists.txt
	controller_manager_tests/CMakeLists.txt