Use Firmware version of sitl_gazebo

sitl_gazebo is cloned as a sub-repo of PX4 Firmware.  Use this version of the code instead of cloning a different version.

May be as simple as using a symbolic link to the version in Firmware.

Reason: 
1. Ensures that the sitl_gazebo repo is in sync with the Firmware repo. 
1. Saves time and disk space by cloning once.
1. No need to maintain forked sitl_gazebo repo on GitHub.