[SIM] "Fully" Automatic Startup

What needs to be achieved:

• Let UE4 run sim_vehicle.py, simple_UDP, and itself using c++ tricks (e.g. we can execute commands and/or create process inside UE4).
• Let UE4 detect whether the ROS Core is up (e.g. exception), and start it when sim start if ROS option is enabled.
• Deconstruct and ending process within one program (UE4 SIM)

Note: Try to do the above tasks one-by-one, top-down (Order of priority). Automatic startup capability is very useful for intensive testing and necessary for publish this SIM as an independent side project.