This commit implements a `set_franka_model_configuration` service. This
service can be used to set the Franka configuration in the Gazebo
simulation. Under the hood, this service calls Gazebo's
'set_model_configuration' service while ensuring that the reported joint
positions stay within joint limits (see
frankaemika#225 for more
information).

This commit ensures the `set_franka_model_configuration` works with the
changes made in 89d2571. It also
simplifies the `SetJointConfiguration.srv` message.