Remove top level layer of config YAML files

[pxalcantara]

Remove the top-level names of the YAML files as suggested in #9,

For the config/motor.yaml I removed the top level motor: like it is done in base.yaml and front_wheel.yaml. This makes the code shorter inside common_macros.urdf.xacro. Maybe this could be used for the other yaml files as well because the description is already in the dictionary name, e.g. motor_props, base_prop, wheel_props. I think having base_prop['base']... and wheel_props['wheel']... is a bit redundant.