[WIP] os module, and executeCommand function

This function will be use everywhere we need to execute an os command.
It's more flexible, if we want to replace the "subprocess.Popen" call in the future for example. Returns the result string.

Todo : 
- [ ] Create the os module
- [ ] Design the executeCommand function
- [ ] Replace code everywhere is needed to use the new function instead of "subprocess" call.