adds tf_prefix argument to imu.gazebo.urdf.xacro

[niniemann]

When creating the urdf description of the robot, use the tf_prefix argument to make the imu plugin publish it's data in the tf_prefix/imu_frame.

Works in nested models, since xacro:args seem to work like global variables (kind of). E.g., our mobipick robot includes the mir, and with no further modifications sets the tf_prefix of the mir's IMU like this:

<param name="robot_description" command="$(find xacro)/xacro --inorder $(find mobipick_description)/urdf/mobipick/mobipick_v1_model.urdf.xacro tf_prefix:=mobipick" />

[mintar]

@ros-pull-request-builder retest this please