Update submodule sitl_gazebo to latest Wed Aug 28 15:06:39 UTC 2019

    - sitl_gazebo in PX4/Firmware (7ee1043): PX4/PX4-SITL_gazebo-classic@22df947
    - sitl_gazebo current upstream: PX4/PX4-SITL_gazebo-classic@a3b8b9c
    - Changes: PX4/PX4-SITL_gazebo-classic@22df947...a3b8b9c

    a3b8b9c 2019-08-18 TSC21 - matrice_100_opt_flow: overwrite model name of the sf10a lidar so the topic name is the expected from gazebo_mavlink_interface
5f5e1ba 2019-08-18 TSC21 - distance sensors: by default, get the topic name from the model name
ebe1e07 2019-01-31 Tully Foote - Add gstreamer plugin to the fpv camera
157c52d 2019-02-04 Tully Foote - enable gst stream from geotagged_camera
d75b7cd 2019-01-25 Tully Foote - turn on video streaming by default
cc8d125 2019-08-17 TSC21 - make it compilable in Gazebo version < 9
dae0fde 2019-08-17 TSC21 - add support for both sonar and lidar rotation determination by transform checks
9915be5 2019-08-15 TSC21 - gazebo_mavlink_interface: rename setSensorOrientation to setMavlinkSensorOrientation
99eb6ee 2019-08-15 TSC21 - move the sensor orientation computation to common.h
fd7b3b1 2019-08-15 TSC21 - gazebo_mavlink_interface: setSensorOrientation: fix args
943ae18 2019-08-13 TSC21 - lidar: adjust sensor orientation computation pipeline
06eb458 2019-04-02 Elia Tarasov - add calculation of distance sensor direction
b45a202 2019-03-28 Elia Tarasov - rename parameter from facing to rotation
4c38627 2019-03-26 Elia Tarasov - add facing parameter to lidar and sonar plugins