Remove executor from config file

Right now the user has to define two things to make the autoscaler create windows VM, the executor and OS for example:

Executor = "winrm"
OS = "windows"

At the moment we only support winrm with Windows and we are going under that assumption as well. It might be best to remove the option of Executor for the user and have that depend on the OS so if it's Windows it's always winRM and if it's Linux it's always ssh. We can always add back this configuration if there is a real use case but I don't think there is a good reason to have this at the moment.

/cc @tmaczukin