Call run_gazebo under drcsim when drcsim is installed

In the environment in which drcsim (gazebo3) is installed, you get the following error when you call gazebo under gazebo_ros.
```
gzserver: symbol lookup error: /opt/ros/hydro/lib/libgazebo_ros_paths_plugin.so: undefined symbol: _ZN3ros7package10getPluginsERKSsS2_RSt6vectorISsSaISsEE
gzclient: symbol lookup error: /opt/ros/hydro/lib/libgazebo_ros_paths_plugin.so: undefined symbol: _ZN3ros7package10getPluginsERKSsS2_RSt6vectorISsSaISsEE
/opt/ros/hydro/lib/gazebo_ros/gazebo: 25: kill: invalid signal number or name: SIGINT
```

This PR checkes whether drcsim is installed or not, and call the correct gazebo script.