Make CARLA start without client by default

[nsubiron]

Currently CARLA waits for the client at start by default. This is confusing for new users as it is not completely clear whether CARLA is working or just hang.

It is a better default to start in "video game" mode, so new users can test it without worrying about the client. People that wants to use the client will need to provide an extra command-line argument, this is also good as they are already supposed to do so (-benchmark -fps=X is the right way to run it with client).