How can i change the initial orientation of a robot when using roslaunch?

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usage: spawn_model [-h] (-urdf | -sdf)
                   (-file FILE_NAME | -param PARAM_NAME | -database MODEL_NAME | -stdin)
                   -model MODEL_NAME [-reference_frame REFERENCE_FRAME]
                   [-gazebo_namespace GAZEBO_NAMESPACE]
                   [-robot_namespace ROBOT_NAMESPACE] [-unpause]
                   [-wait MODEL_NAME] [-x X] [-y Y] [-z Z] [-R R] [-P P]
                   [-Y Y] [-J JOINT_NAME JOINT_POSITION] [-package_to_model]
                   [-b]

R, P, and Y stand for roll pitch and yaw. Yaw is what you want. Hence:

<node name="spawn_urdf" pkg="gazebo_ros" type="spawn_model"
args="-param robot_description -urdf -x -0.5 -y 1 -z 0.05 -Y 3.1416 -model pepper_MP" />