Add Gazebo plane config

ROMFS/px4fmu_common/mixers/plane_sitl.main.mix

+Mixer for SITL plane, using the VTOL airframe for now
+=========================================================
+
+Z:
+
+Z:
+
+Z:
+
+Z:
+
+# mixer for the elevons
+M: 2
+O:      10000  10000   0 -10000  10000
+S: 0 0  5000   5000    0 -10000  10000
+S: 0 1 -5000 -5000     0 -10000  10000
+
+M: 2
+O:       10000   10000   0 -10000  10000
+S: 0 0   5000    5000    0 -10000  10000
+S: 0 1   5000    5000    0 -10000  10000
+
+# mixer for the pusher/puller throttle
+M: 1
+O:      10000  10000      0 -10000  10000
+S: 0 3      0  20000 -10000 -10000  10000