Only show QGC virtual joystick when simulating a single vehicle

[JacopoPan]

Motivation:

• Quads and VTOLs require different throttle centering
• With multiple vehicles switching vehicle focus in QGC is interpreted as RC loss/reconnection from the autopilot SITL

TODOs

• test missions

Notes:

• no virtual joystick makes ArduCopter "LOITER" mode not hold altitude when exiting/aborting an Offboard action, using "BRAKE" instead (while keeping "LOITER" for ArduPlane)
• this PR disables the virtual joystick for multi-drone simulation, this makes PX4 vehicle appear in QGC in a yellow warning state for "No RC Input", this warning is harmless and can be removed by setting COM_RC_IN_MODE to 4

Alternatively one could add to Dockerfile.simulation

RUN mkdir /airframes
# Copy all airframes and create no-RC variants with COM_RC_IN_MODE=4
RUN find /temp_folder -type f -regex '.*/[0-9]+_.*' | while read -r filepath; do \
        filename=$(basename "$filepath"); \
        cp "$filepath" "/airframes/$filename"; \
        airframe_id=${filename%%_*}; \
        vehicle_name=${filename#*_}; \
        no_rc_airframe_id=$((airframe_id + 1000)); \
        no_rc_filename="${no_rc_airframe_id}_${vehicle_name}_no_rc"; \
        cp "$filepath" "/airframes/$no_rc_filename"; \
        echo "param set-default COM_RC_IN_MODE 4" >> "/airframes/$no_rc_filename"; \
    done

and to `simulation.yml.erb

    if autopilot == 'px4'
      if num_quads + num_vtols == 1
        autostart = is_quad ? 5140 : 5141
      else
        autostart = is_quad ? 6140 : 6141
      end

but it seems needlessly complicated