Fixes the following error (occurring since 61d18f2)
```
[FATAL] ros.moveit_ros_planning.trajectory_execution_manager:
  Exception while loading controller manager 'robot':
  According to the loaded plugin descriptions the class robot
  with base class type moveit_controller_manager::MoveItControllerManager does not exist.
```

As we introduced `pass_all_args="true"`, the value of the argument
`moveit_controller_manager` was the robot name.

There are 3 basic MoveIt controller manager plugins:
- fake = `moveit_fake_controller_manager::MoveItFakeControllerManager`
  Used in demo.launch. Doesn't really control the robot, but just
  interpolates between via points. Allows these execution_types:
  - via points: just jumps to the via points
  - interpolate: linearly interpolates between via points (default)
  - last point: jumps to the final trajectory point (used for fast execution testing)

- ros_control = `moveit_ros_control_interface::MoveItControllerManager`
  Interfaces to ros_control controllers.

- simple = `moveit_simple_controller_manager/MoveItSimpleControllerManager`
  Interfaces to action servers for `FollowJointTrajectory` and/or `GripperCommand`
  that in turn interface to the low-level robot controllers (typically based on ros_control)

However, so far move_group.launch distinguished between `fake` and `robot` only.
The argument moveit_controller_manager now allows switching between all 3 variants.
Adding more *_moveit_controller_manager.launch files allows for an extension of this scheme.

This will facilitate re-use of demo.launch.

Only unpause simulation when robot model was loaded.
This ensures that the initial pose is actually held.

... to clarify that this parameter is only used for fake controllers

There is no means to declare the planning_plugin as an arg first.